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>

backend/alembic/env.py

@@ -0,0 +1,49 @@
+from logging.config import fileConfig
+
+from alembic import context
+from sqlalchemy import engine_from_config, pool
+from sqlmodel import SQLModel
+
+from app.config import settings
+import app.models.models  # noqa: F401 — register all tables on the metadata
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# Single source of truth for the DB URL: the app settings.
+config.set_main_option("sqlalchemy.url", settings.DATABASE_URL)
+
+target_metadata = SQLModel.metadata
+
+
+def run_migrations_offline() -> None:
+    context.configure(
+        url=config.get_main_option("sqlalchemy.url"),
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    connectable = engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    with connectable.connect() as connection:
+        context.configure(
+            connection=connection, target_metadata=target_metadata
+        )
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()