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>

api/pyproject.toml

@@ -1,6 +1,6 @@
 [build-system]
 requires = ["setuptools>=68"]
-build-backend = "setuptools.backends.legacy:build"
+build-backend = "setuptools.build_meta"
 
 [project]
 name = "reactbin-api"
@@ -35,6 +35,8 @@ ignore = []
 
 [tool.pytest.ini_options]
 asyncio_mode = "auto"
+asyncio_default_fixture_loop_scope = "session"
+asyncio_default_test_loop_scope = "session"
 testpaths = ["tests"]
 
 [tool.setuptools.packages.find]