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/routers/images.py

@@ -167,7 +167,7 @@ async def upload_image(
     if tag_names:
         tag_repo = TagRepository(db)
         await tag_repo.attach_tags(image, tag_names)
-        await db.refresh(image, ["image_tags"])
+        image = await image_repo.reload_with_tags(image.id)
 
     return _image_to_dict(image, duplicate=False)
 
@@ -248,7 +248,7 @@ async def update_image_tags(
         )
 
     await tag_repo.replace_tags_on_image(image, tag_names)
-    await db.refresh(image, ["image_tags"])
+    image = await image_repo.reload_with_tags(image.id)
     return _image_to_dict(image)