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/models/models.py

@@ -1,12 +1,17 @@
 import enum
 from datetime import date, datetime
-
-from app.timeutil import utcnow
 from typing import Optional
 
 from sqlalchemy import JSON, Column, UniqueConstraint
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlmodel import Field, Relationship, SQLModel
 
+from app.timeutil import utcnow
+
+# Live DBs store usersettings.data as jsonb; keep plain JSON elsewhere and on
+# SQLite (tests).
+JSON_OR_JSONB = JSON().with_variant(JSONB(), "postgresql")
+
 
 class RecurringItemType(str, enum.Enum):
     INCOME = "INCOME"
@@ -144,7 +149,9 @@ class TransactionBase(SQLModel):
     bank: Bank = Bank.BAC
     notes: Optional[str] = None
     category_id: Optional[int] = Field(default=None, foreign_key="category.id")
-    deferred_to_next_cycle: bool = Field(default=False)
+    deferred_to_next_cycle: bool = Field(
+        default=False, sa_column_kwargs={"server_default": "false"}
+    )
 
 
 class Transaction(TransactionBase, table=True):
@@ -227,7 +234,7 @@ class UserSettings(SQLModel, table=True):
     key: str = Field(index=True, unique=True, default="default")
     data: dict = Field(
         default_factory=dict,
-        sa_column=Column(JSON, nullable=False, server_default="{}"),
+        sa_column=Column(JSON_OR_JSONB, nullable=False, server_default="{}"),
     )
     updated_at: datetime = Field(default_factory=utcnow)