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/main.py

@@ -1,6 +1,8 @@
 from contextlib import asynccontextmanager
 
-from fastapi import FastAPI
+from fastapi import FastAPI, Request
+from fastapi.exceptions import HTTPException
+from fastapi.responses import JSONResponse
 
 from app.config import get_settings
 from app.database import get_engine, get_session_factory, Base
@@ -21,6 +23,13 @@ async def lifespan(application: FastAPI):
 app = FastAPI(title="Reactbin API", version="1.0.0", lifespan=lifespan)
 
 
+@app.exception_handler(HTTPException)
+async def http_exception_handler(request: Request, exc: HTTPException):
+    if isinstance(exc.detail, dict):
+        return JSONResponse(status_code=exc.status_code, content=exc.detail)
+    return JSONResponse(status_code=exc.status_code, content={"detail": exc.detail})
+
+
 @app.get("/api/v1/health")
 async def health():
     return {"status": "ok"}