feat: add Quart app factory, config loader, and /api/health endpoint

Config reads from env vars (12-factor); .env.example documents defaults.
Health endpoint is liveness-only (no DB/Redis touch). Test added for CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/extensions.py

@@ -0,0 +1,14 @@
+"""Singleton extension instances; bound to the app in create_app()."""
+
+from sqlalchemy.ext.asyncio import AsyncEngine, async_sessionmaker, create_async_engine
+
+from .config import get_config
+
+
+def make_engine() -> AsyncEngine:
+    cfg = get_config()
+    return create_async_engine(cfg.database_url, pool_pre_ping=True, future=True)
+
+
+def make_session_factory(engine: AsyncEngine) -> async_sessionmaker:
+    return async_sessionmaker(engine, expire_on_commit=False)