refactor(assets): database layer — split queries into modules and merge migrations

- Split monolithic queries.py into modular query modules (asset, asset_reference, common, tags)
- Absorb bulk_ops.py and tags.py into query modules
- Merge migrations 0002-0005 into single migration (0002_merge_to_asset_references)
- Update models.py (merge AssetInfo/AssetCacheState into AssetReference)
- Enable SQLite foreign key enforcement
- Add comprehensive query-layer tests

Co-authored-by: Amp <amp@ampcode.com>
Amp-Thread-ID: https://ampcode.com/threads/T-019c917d-82b5-7448-a04f-9cd59c69d0a2

app/database/db.py

@@ -14,7 +14,7 @@ try:
     from alembic.config import Config
     from alembic.runtime.migration import MigrationContext
     from alembic.script import ScriptDirectory
-    from sqlalchemy import create_engine
+    from sqlalchemy import create_engine, event
     from sqlalchemy.orm import sessionmaker
 
     _DB_AVAILABLE = True
@@ -75,6 +75,13 @@ def init_db():
 
     # Check if we need to upgrade
     engine = create_engine(db_url)
+
+    # Enable foreign key enforcement for SQLite
+    @event.listens_for(engine, "connect")
+    def set_sqlite_pragma(dbapi_connection, connection_record):
+        cursor = dbapi_connection.cursor()
+        cursor.execute("PRAGMA foreign_keys=ON")
+        cursor.close()
     conn = engine.connect()
 
     context = MigrationContext.configure(conn)