fix(db): declare locale on CommandConfig model + defensive migration

Startup was crashing on fresh databases because:
- init_db() calls SQLModel.metadata.create_all(), which builds tables
  from the model classes. CommandConfig didn't declare `locale`, so
  the created command_config table lacked the column.
- The seeder then issued INSERTs that included locale='en', causing
  `OperationalError: table command_config has no column named locale`.

The legacy migration #6 in migrate_schema creates command_config WITH
locale via raw SQL, so upgraded databases worked. Only fresh installs
broke.

Fix:
- Add `locale: str = Field(default='en')` to CommandConfig model so
  create_all() produces a consistent schema.
- Add a defensive ALTER TABLE ... ADD COLUMN locale in migrate_schema's
  else-branch, so any existing command_config table missing the column
  (from a broken v0.1.0 install) is backfilled on next startup.

packages/server/src/notify_bridge_server/database/migrations.py

@@ -614,6 +614,13 @@ async def migrate_entity_refactor(engine: AsyncEngine) -> None:
                 ")"
             ))
             logger.info("Created command_config table")
+        else:
+            # Backfill locale column for tables created before locale was on the model
+            if not await _has_column(conn, "command_config", "locale"):
+                await conn.execute(
+                    text("ALTER TABLE command_config ADD COLUMN locale TEXT DEFAULT 'en'")
+                )
+                logger.info("Added locale column to command_config table")
 
         # ------------------------------------------------------------------
         # 7. Create command_tracker table