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>

.gitignore

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 # Claude Code local state
 .claude/
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