refactor(assets): split queries.py into modular query modules

Split the ~1000 line app/assets/database/queries.py into focused modules:

- queries/asset.py - Asset entity queries (asset_exists_by_hash, get_asset_by_hash)
- queries/asset_info.py - AssetInfo queries (~15 functions)
- queries/cache_state.py - AssetCacheState queries (list_cache_states_by_asset_id,
  pick_best_live_path, prune_orphaned_assets, fast_db_consistency_pass)
- queries/tags.py - Tag queries (8 functions including ensure_tags_exist,
  add/remove tag functions, list_tags_with_usage)
- queries/__init__.py - Re-exports all public functions for backward compatibility

Also adds comprehensive unit tests using in-memory SQLite:
- tests-unit/assets_test/queries/conftest.py - Session fixture
- tests-unit/assets_test/queries/test_asset.py - 5 tests
- tests-unit/assets_test/queries/test_asset_info.py - 23 tests
- tests-unit/assets_test/queries/test_cache_state.py - 8 tests
- tests-unit/assets_test/queries/test_metadata.py - 12 tests for _apply_metadata_filter
- tests-unit/assets_test/queries/test_tags.py - 23 tests

All 71 unit tests pass. Existing integration tests unaffected.

Amp-Thread-ID: https://ampcode.com/threads/T-019c24bb-475b-7442-9ff9-8288edea3345
Co-authored-by: Amp <amp@ampcode.com>

app/assets/database/queries/asset.py

@@ -0,0 +1,31 @@
+import sqlalchemy as sa
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from app.assets.database.models import Asset
+
+
+def asset_exists_by_hash(
+    session: Session,
+    *,
+    asset_hash: str,
+) -> bool:
+    """
+    Check if an asset with a given hash exists in database.
+    """
+    row = (
+        session.execute(
+            select(sa.literal(True)).select_from(Asset).where(Asset.hash == asset_hash).limit(1)
+        )
+    ).first()
+    return row is not None
+
+
+def get_asset_by_hash(
+    session: Session,
+    *,
+    asset_hash: str,
+) -> Asset | None:
+    return (
+        session.execute(select(Asset).where(Asset.hash == asset_hash).limit(1))
+    ).scalars().first()