Adopt Alembic: autogenerated baseline, stamp-or-upgrade startup

The baseline (c3da001a0eb3) was autogenerated against an empty Postgres
and schema-diffed against the live dev schema until column-identical
(two model fixes fell out: deferred_to_next_cycle now declares its
server_default, usersettings.data is JSONB on Postgres to match the
live type). Startup replaces init_db/run_migrations with
run_alembic_upgrade(): pre-Alembic databases that already match the
baseline are adopted via stamp; fresh databases build from the
migration. The section is serialized with a pg advisory lock because
prod uvicorn runs 2 workers, each executing the lifespan. Verified:
adoption + idempotent re-run on the dev DB, fresh 13-table build on an
empty DB, full container boot, 55/55 tests. (ARCH-02, BE-19)

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

CLAUDE.md

@@ -17,6 +17,27 @@ The backend refuses to boot without `SECRET_KEY`, `ADMIN_USERNAME`, and
 `ADMIN_PASSWORD` (no insecure defaults). Copy `.env.example` to `.env` and fill
 it in; the dev docker-compose reads it.
 
+## Database Migrations
+
+Schema changes go through Alembic — never edit the schema by hand and never
+reset the prod DB. Workflow:
+
+```bash
+cd backend
+# 1. Edit app/models/models.py
+# 2. Autogenerate against the dev DB, then REVIEW the generated file
+DATABASE_URL='postgresql://wealthy_user:wealthy_pass@localhost:5433/wealthysmart' \
+  .venv/bin/alembic revision --autogenerate -m "describe the change"
+# 3. Migrations apply automatically at app startup (run_alembic_upgrade)
+```
+
+## Tests
+
+```bash
+cd backend && .venv/bin/python -m pytest tests/ -q   # backend (55+ tests)
+cd frontend && pnpm typecheck                        # frontend
+```
+
 ## Local Docker
 
 ```bash