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/app/repositories/image_repo.py

@@ -79,6 +79,15 @@ class ImageRepository:
         result = await self._session.execute(paginated)
         return result.scalars().all(), total
 
+    async def reload_with_tags(self, image_id: uuid.UUID) -> Image:
+        result = await self._session.execute(
+            select(Image)
+            .where(Image.id == image_id)
+            .options(selectinload(Image.image_tags).selectinload(ImageTag.tag))
+            .execution_options(populate_existing=True)
+        )
+        return result.scalar_one()
+
     async def delete(self, image: Image) -> None:
         await self._session.delete(image)
         await self._session.flush()