feat: add DB inline content cache for app asset draft files

Introduce app_asset_contents table as a read-through cache over S3 for
text-like asset files (e.g. .md skill documents). This eliminates N
individual S3 fetches during SkillBuilder builds — bulk_load pulls all
content in a single SQL query with S3 fallback on miss.

Key components:
- CachedContentAccessor: DB-first read / dual-write / S3 fallback
- AssetContentService: static DB operations (get, get_many, upsert, delete)
- should_mirror(): single source of truth for extension-based policy
- Alembic migration for app_asset_contents table

Modified callers:
- SkillBuilder uses accessor.bulk_load() instead of per-node S3 reads
- AppAssetService.get/update_file_content route through accessor
- delete_node cleans both DB cache and S3
- draft_app_assets_initializer uses should_mirror() instead of hardcoded .md

api/tests/unit_tests/core/app_assets/test_storage.py

@@ -89,21 +89,6 @@ def test_asset_paths_draft_validation():
         AssetPaths.draft(tenant_id=tenant_id, app_id=app_id, node_id="not-a-uuid")
 
 
-def test_asset_paths_resolved_requires_node_id():
-    """Test that AssetPaths.resolved() requires a valid node_id."""
-    tenant_id = str(uuid4())
-    app_id = str(uuid4())
-    assets_id = str(uuid4())
-
-    # Missing node_id should raise
-    with pytest.raises(TypeError):
-        AssetPaths.resolved(tenant_id, app_id, assets_id)  # type: ignore[call-arg]
-
-    # Invalid node_id should raise
-    with pytest.raises(ValueError, match="node_id must be a valid UUID"):
-        AssetPaths.resolved(tenant_id, app_id, assets_id, node_id="not-a-uuid")
-
-
 # --- Storage key format tests (must match existing paths exactly) ---