docs(deploy): note the /data uid/gid-mismatch trap on first start

Hit this deploying to the real Unraid host: the container runs as a fixed
non-root uid/gid (999), not root and not Unraid's usual nobody:users
(99:100). A freshly-created appdata directory is owned by nobody:users with
no write access for anyone else, so the container starts but uvicorn fails
immediately with "unable to open database file" — not obvious from the
error alone, worth documenting once rather than re-debugging it later.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01R2ZKeWkZV7ehf7fivrAkkG

deploy/README.md

@@ -63,6 +63,13 @@ the same values, there's no separate migration-time config anymore (docs/DECISIO
 |---|---|
 | `/data` | The SQLite database file. Will also hold the content-addressed blob store once Phase 1 builds ingestion. This is the only thing that needs backing up. |
 
+The container runs as a fixed non-root user (uid/gid `999`), not root and not Unraid's usual
+`nobody:users` (99:100). If `/data`'s host directory doesn't already exist, Docker/Unraid creates
+it owned by `nobody:users` with no write access for other users — the container starts, but
+uvicorn fails immediately with `sqlite3.OperationalError: unable to open database file`, since it
+can't create the SQLite file inside a directory it can't write to. Fix once, before first start:
+`chown -R 999:999 <host path>` (e.g. `/mnt/user/appdata/velodrome` on Unraid).
+
 ## Unraid
 
 Import `unraid-template.xml` from the Docker tab's "Add Container" template picker — it exposes