Fix 502 after update: safe startup, single DB init, healthcheck

Co-authored-by: Cursor <cursoragent@cursor.com>

app/routes.py

@@ -25,6 +25,15 @@ from app.upload_service import process_uploads
 bp = Blueprint("main", __name__)
 
 
+@bp.route("/health")
+def health():
+    try:
+        db.session.execute(db.text("SELECT 1"))
+        return {"status": "ok"}, 200
+    except Exception as exc:
+        return {"status": "error", "detail": str(exc)}, 503
+
+
 @bp.route("/")
 def index():
     photos = Photo.query.order_by(Photo.created_at.desc()).limit(24).all()