refactor(api): move from Postgres+RLS to single-engine SQLite

Reverses a shipped, tested, merged decision (D4/PR #2) rather than building
on it — see docs/DECISIONS.md D15 for the full record: what was rejected
(Postgres as a second container; Postgres+PostGIS bundled inside the single
container via a supervisor), what this costs (no database-level RLS, no
PostGIS, procrastinate needs replacing — all stated as a concern before this
was decided, and reaffirmed anyway, which is the user's call to make about
their own instance).

The one invariant-critical consequence: isolation between users now rests
entirely on the repository-layer scope (db.py's `Scope.select()`), not two
layers. CLAUDE.md's invariant #4 is revised accordingly. This is not a
downgrade-and-hope — `Scope` is built so an unfiltered query against a
user-owned table is structurally harder to write than a scoped one (there is
no method on `Scope` that returns one), and
tests/test_auth.py::test_scoped_session_blocks_cross_user_reads replaces the
old RLS proof with the same empirical standard: it doesn't trust the query
builder filters correctly because the code reads correctly, it registers two
real users and checks.
test_unscoped_session_can_see_every_user_when_misused is the deliberately
alarming companion — it demonstrates exactly what a reviewer must now catch,
since nothing else will.

Six real, non-obvious SQLite behaviours found and fixed by actually running
this against a real file, not assumed from docs:

- Foreign keys, ON DELETE CASCADE included, are OFF by default per
  connection — deleting a user silently left orphaned sessions/api_tokens,
  no error either way. Fixed with PRAGMA foreign_keys=ON on every connect.
- Transactions default to DEFERRED, which only takes a write lock on the
  first actual write — a real check-then-act race for invite redemption
  (two concurrent redemptions could both read used_count < max_uses as true
  before either commits). Fixed by disabling the driver's implicit BEGIN and
  issuing BEGIN IMMEDIATE ourselves — SQLAlchemy's own documented recipe for
  this, not improvised.
- DateTime(timezone=True) does NOT round-trip tzinfo on SQLite — a tz-aware
  datetime goes in, a naive one comes back out, and every
  `expires_at < datetime.now(UTC)` comparison in auth/service.py then raises
  TypeError. Fixed once at the Base level with a UTCDateTime TypeDecorator
  rather than per-column.
- Uuid(as_uuid=True) stores as 32-char hex with NO hyphens on SQLite, not
  str(uuid)'s hyphenated form. A test fixture that raw-inserted the
  hyphenated form left rows the ORM's own later UPDATE (via
  invite.used_count += 1's autoflush) could never match by primary key,
  updating zero rows and raising StaleDataError. Fixed by using .hex to
  match exactly what the ORM itself writes.
- BEGIN IMMEDIATE applies to every transaction, reads included — a
  long-lived test fixture that autobegins a transaction via a bare read and
  never explicitly closes it holds SQLite's exclusive write lock for the
  rest of the test, and a later scoped_session() call fails with "database
  is locked". Not an app-code bug (every real session block closes cleanly
  on exit), but real enough to document since the next person writing a
  test against the db_auth fixture will hit it too.
- Python's sqlite3 module deprecates its own implicit datetime adapter as of
  3.12 — silent today, warns on every raw-SQL datetime bind. Only ever hit
  test fixture code (the ORM path never uses it, confirmed by running the
  ORM-only health test with warnings promoted to errors and it stayed
  clean); fixed there with an explicit .isoformat() rather than left for a
  future Python version to turn into a real failure.

Also, since with_for_update() silently no-ops on SQLite (confirmed —
SQLAlchemy emits no SQL for it, no error either) rather than actually
locking anything: removed it from register()'s invite-redemption query and
corrected the comment to attribute the concurrency guarantee to BEGIN
IMMEDIATE, where it now actually lives.

One PR, not several, for the same reason PR #2 was: the migration, the
models, db.py, and the docs recording why are five views of one decision —
splitting them wouldn't make review easier, just disconnected. 552
insertions / 548 deletions across 17 files, most of it necessarily touching
what PR #2 shipped rather than net-new code.

Deliberately deferred, not solved here: PostGIS's replacement for spatial
storage, procrastinate's replacement for background jobs, and the
EXCLUDE USING gist constraint's replacement for component_installs — none
of those tables exist yet (Phase 1-2), so none of it is broken, and
docs/DECISIONS.md D15 records exactly what each future phase needs to
decide before it can be built. .gitea/workflows/deploy pipeline (PR #4,
built for the old 3-container Postgres compose stack) was closed as
superseded rather than merged; the single-container image build is
follow-up work, not part of this change.

Verified: ruff check, ruff format --check, and mypy --strict all clean.
13/13 pytest passing against a real SQLite file, including with
DeprecationWarning promoted to an error (confirms the sqlite3 adapter
deprecation fix actually holds, not just that it's quiet by default). Full
alembic upgrade -> downgrade -1 -> upgrade cycle run clean. alembic check
clean with no include_object filter needed at all now (SQLite starts with
nothing but what our own migrations create — no PostGIS/TIGER noise to
filter out in the first place). CI's exact migration command sequence
reproduced locally end to end before touching the workflow file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/api/tests/conftest.py

@@ -1,13 +1,16 @@
 """Test fixtures.
 
-Runs the real Alembic migrations once per session against a real Postgres (never mocked — see
-CLAUDE.md's test policy), then truncates the identity tables between tests for isolation. This
-deliberately exercises the exact same migration path CI's `migrations` job and a real deploy use,
-not a parallel test-only schema-creation shortcut.
+Runs the real Alembic migrations once per session against a real SQLite file (never mocked — see
+CLAUDE.md's test policy — and never `:memory:`, which gives each separate connection its own
+isolated database rather than one shared one), then deletes all rows between tests for isolation.
+This deliberately exercises the exact same migration path CI's `migrations` job and a real deploy
+use, not a parallel test-only schema-creation shortcut.
 """
 
 import os
+import tempfile
 from collections.abc import AsyncIterator
+from pathlib import Path
 
 import pytest
 from alembic.config import Config
@@ -18,29 +21,12 @@ from alembic import command
 
 os.environ.setdefault("VELODROME_ENVIRONMENT", "test")
 
-# Test-only role passwords. Never used outside this process; the migration requires them to be
-# set explicitly (see alembic/versions/0001_baseline.py::_require_password) rather than default
-# to anything, on purpose — that's the same rule for a real deploy, just satisfied differently.
-_APP_PW = "test-only-app-password"
-_AUTH_PW = "test-only-auth-password"
+_tmp_dir = tempfile.mkdtemp(prefix="velodrome-test-")
+_db_path = Path(_tmp_dir) / "test.db"
+os.environ.setdefault("VELODROME_DATABASE_URL", f"sqlite+aiosqlite:///{_db_path}")
 
-os.environ.setdefault(
-    "VELODROME_DATABASE_URL_MIGRATE",
-    "postgresql+asyncpg://postgres:postgres@localhost:5432/velodrome_test",
-)
-os.environ.setdefault("VELODROME_DB_APP_PASSWORD", _APP_PW)
-os.environ.setdefault("VELODROME_DB_AUTH_PASSWORD", _AUTH_PW)
-os.environ.setdefault(
-    "VELODROME_DATABASE_URL_APP",
-    f"postgresql+asyncpg://velodrome_app:{_APP_PW}@localhost:5432/velodrome_test",
-)
-os.environ.setdefault(
-    "VELODROME_DATABASE_URL_AUTH",
-    f"postgresql+asyncpg://velodrome_auth:{_AUTH_PW}@localhost:5432/velodrome_test",
-)
-
-# Settings/engines must not be constructed before the env vars above are set, so these imports
-# are deliberately below the os.environ.setdefault block, not at module top.
+# Settings/engines must not be constructed before the env var above is set, so these imports are
+# deliberately below it, not at module top.
 import httpx  # noqa: E402
 
 from velodrome.app import app  # noqa: E402
@@ -51,7 +37,6 @@ from velodrome.db import auth_session  # noqa: E402
 def _run_migrations() -> None:
     cfg = Config(os.path.join(os.path.dirname(__file__), "..", "alembic.ini"))
     cfg.set_main_option("script_location", os.path.join(os.path.dirname(__file__), "..", "alembic"))
-    command.downgrade(cfg, "base")  # clean slate even on a reused test database
     command.upgrade(cfg, "head")
 
 
@@ -60,9 +45,7 @@ async def _clean_tables() -> AsyncIterator[None]:
     yield
     async with auth_session() as db:
         async with db.begin():
-            # DELETE, not TRUNCATE — velodrome_auth deliberately isn't granted TRUNCATE in
-            # production (see the migration's comment), and using the same privilege level in
-            # tests as in prod is the point. Children before parents for the FK constraints.
+            # Children before parents for the FK constraints.
             for table in ("sessions", "api_tokens", "invites", "users"):
                 await db.execute(text(f"DELETE FROM {table}"))