Fix build failures and get all 45 tests passing

Build fixes:
- ui/Dockerfile: npm install instead of npm ci (no lockfile)
- api/pyproject.toml: setuptools.build_meta instead of setuptools.backends.legacy:build
- api/Dockerfile: install curl so the Docker healthcheck doesn't always fail
- docker-compose.yml: add start_period: 30s to API healthcheck

Test fixes:
- pyproject.toml: asyncio_default_fixture_loop_scope/test_loop_scope = session to
  prevent asyncpg connections being used across different event loops
- conftest.py: loop_scope="session" on session-scoped engine fixture
- main.py: custom HTTPException handler to flatten dict details to top level
  (FastAPI wraps dict details as {"detail": {...}} by default)
- test_upload.py: use env var + cache_clear() to override max_upload_bytes since
  monkeypatch can't reach past @lru_cache and already-imported references
- image_repo.py: add reload_with_tags() with populate_existing=True to force
  SQLAlchemy to repopulate the identity-map object after tag mutations
- images.py: use reload_with_tags() instead of db.refresh(image, ["image_tags"])
  which only loaded ImageTag rows without their .tag sub-relationship, causing
  MissingGreenlet on any access to image.tags after attach/replace operations

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -66,6 +66,7 @@ services:
       interval: 10s
       timeout: 5s
       retries: 5
+      start_period: 30s
 
   ui:
     build: