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/uv.lock

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@@ -1170,9 +1139,9 @@ name = "velodrome"
 version = "0.1.0"
 source = { editable = "." }
 dependencies = [
+    { name = "aiosqlite" },
     { name = "alembic" },
     { name = "argon2-cffi" },
-    { name = "asyncpg" },
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     { name = "pydantic", extra = ["email"] },
     { name = "pydantic-settings" },
@@ -1192,9 +1161,9 @@ dev = [
 
 [package.metadata]
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