feat(db): add auto-migration to create missing columns on existing tables

SQLModel's create_all only creates new tables but does not alter existing
ones to add new columns. This introduces a _auto_migrate_sync helper that
inspects the live schema and issues ALTER TABLE … ADD COLUMN for any
column present in the model but absent from the database.

Only column additions are supported (safe, non-destructive). Column
renames, type changes, and removals are intentionally not handled.

Author: RC-CHN

pkgs/bay/app/db/session.py

@@ -5,6 +5,8 @@
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
 
+import structlog
+from sqlalchemy import inspect, text
 from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
 from sqlalchemy.orm import sessionmaker
 from sqlmodel import SQLModel
@@ -14,6 +16,8 @@
 import app.models  # noqa: F401
 from app.config import get_settings
 
+logger = structlog.get_logger()
+
 # Lazy initialization - engine created on first use
 _engine = None
 _async_session_factory = None
@@ -44,6 +48,50 @@ def _get_session_factory():
     return _async_session_factory
 
 
+def _auto_migrate_sync(conn) -> None:
+    """Auto-migrate existing tables to add missing columns.
+
+    SQLModel's create_all only creates new tables — it does NOT alter
+    existing tables to add new columns.  This helper inspects the live
+    schema and issues ALTER TABLE … ADD COLUMN for any column present
+    in the model but absent from the database.
+
+    Only supports column *additions* (safe, non-destructive).
+    Does NOT handle column renames, type changes, or removals.
+    """
+    inspector = inspect(conn)
+    existing_tables = inspector.get_table_names()
+
+    for table in SQLModel.metadata.sorted_tables:
+        if table.name not in existing_tables:
+            # Table will be created by create_all — skip
+            continue
+
+        existing_cols = {c["name"] for c in inspector.get_columns(table.name)}
+
+        for col in table.columns:
+            if col.name in existing_cols:
+                continue
+
+            # Build column type DDL string
+            col_type = col.type.compile(dialect=conn.dialect)
+            nullable = "NULL" if col.nullable else "NOT NULL"
+            default_clause = ""
+            if col.server_default is not None:
+                default_clause = f" DEFAULT {col.server_default.arg}"
+            elif col.nullable:
+                default_clause = " DEFAULT NULL"
+
+            ddl = f"ALTER TABLE {table.name} ADD COLUMN {col.name} {col_type} {nullable}{default_clause}"
+            logger.info(
+                "db.auto_migrate.add_column",
+                table=table.name,
+                column=col.name,
+                ddl=ddl,
+            )
+            conn.execute(text(ddl))
+
+
 async def init_db() -> None:
     """Initialize database tables.
 
@@ -52,6 +100,9 @@ async def init_db() -> None:
     """
     engine = _get_engine()
     async with engine.begin() as conn:
+        # Auto-migrate existing tables first (add missing columns)
+        await conn.run_sync(_auto_migrate_sync)
+        # Then create any entirely new tables
         await conn.run_sync(SQLModel.metadata.create_all)