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/app/db.py

@@ -1,79 +1,48 @@
-from sqlalchemy import text
-from sqlmodel import SQLModel, Session, create_engine
+from pathlib import Path
+
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import inspect, text
+from sqlmodel import Session, create_engine
 
 from app.config import settings
 
 engine = create_engine(settings.DATABASE_URL)
 
-
-def init_db():
-    SQLModel.metadata.create_all(engine)
+_MIGRATION_LOCK_KEY = 727274  # app-wide advisory lock id, arbitrary constant
 
 
-def run_migrations():
-    """Run idempotent schema migrations for columns added after initial create."""
+def _alembic_config() -> Config:
+    backend_dir = Path(__file__).resolve().parent.parent
+    cfg = Config(str(backend_dir / "alembic.ini"))
+    cfg.set_main_option("script_location", str(backend_dir / "alembic"))
+    return cfg
+
+
+def run_alembic_upgrade() -> None:
+    """Bring the schema to head at startup.
+
+    Databases that predate Alembic (prod, existing dev volumes) already match
+    the baseline revision — the baseline was autogenerated from, and
+    schema-diffed against, exactly that schema — so they are adopted by
+    stamping instead of re-running DDL. Serialized with a Postgres advisory
+    lock because prod uvicorn runs 2 workers, each executing the lifespan.
+    """
+    cfg = _alembic_config()
+    is_postgres = engine.dialect.name == "postgresql"
     with engine.connect() as conn:
+        if is_postgres:
+            conn.execute(text(f"SELECT pg_advisory_lock({_MIGRATION_LOCK_KEY})"))
         try:
-            conn.execute(
-                text(
-                    "ALTER TABLE transaction ADD COLUMN IF NOT EXISTS deferred_to_next_cycle BOOLEAN NOT NULL DEFAULT false"
+            insp = inspect(conn)
+            if not insp.has_table("alembic_version") and insp.has_table("transaction"):
+                command.stamp(cfg, "head")
+            command.upgrade(cfg, "head")
+        finally:
+            if is_postgres:
+                conn.execute(
+                    text(f"SELECT pg_advisory_unlock({_MIGRATION_LOCK_KEY})")
                 )
-            )
-            conn.commit()
-        except Exception:
-            conn.rollback()
-
-        try:
-            conn.execute(text("ALTER TYPE currency ADD VALUE IF NOT EXISTS 'EUR'"))
-            conn.commit()
-        except Exception:
-            conn.rollback()
-
-        try:
-            conn.execute(
-                text("ALTER TYPE transactiontype ADD VALUE IF NOT EXISTS 'SALARY'")
-            )
-            conn.commit()
-        except Exception:
-            conn.rollback()
-
-        try:
-            conn.execute(
-                text(
-                    """
-                    CREATE TABLE IF NOT EXISTS savingsaccrual (
-                        id SERIAL PRIMARY KEY,
-                        year INTEGER NOT NULL,
-                        month INTEGER NOT NULL,
-                        memp_amount DOUBLE PRECISION NOT NULL DEFAULT 200000,
-                        mpat_amount DOUBLE PRECISION NOT NULL DEFAULT 200000,
-                        trigger_transaction_id INTEGER,
-                        applied_at TIMESTAMP NOT NULL DEFAULT NOW(),
-                        notes TEXT,
-                        CONSTRAINT savingsaccrual_year_month_key UNIQUE (year, month)
-                    )
-                    """
-                )
-            )
-            conn.commit()
-        except Exception:
-            conn.rollback()
-
-        try:
-            conn.execute(
-                text(
-                    """
-                    INSERT INTO savingsaccrual (year, month, memp_amount, mpat_amount, notes)
-                    VALUES
-                      (2026, 2, 200000, 200000, 'Seeded: historical baseline'),
-                      (2026, 3, 200000, 200000, 'Seeded: historical baseline')
-                    ON CONFLICT (year, month) DO NOTHING
-                    """
-                )
-            )
-            conn.commit()
-        except Exception:
-            conn.rollback()
 
 
 def get_session():