fix: improve error handling for Cloudflare-protected clone sources

- Detect HTML responses from FlareSolverr and throw descriptive error
  instead of letting JSON.parse fail with cryptic 'Unexpected token' error
- Add try/catch to clone/route.ts GET handler to return 502 with
  clear message instead of Internal Server Error 500
- Add FLARESOLVERR_URL to .env

src/lib/services/clone-import.ts

@@ -106,6 +106,11 @@ export async function fetchSourceFurnidata(
 	} catch {
 		body = await fetchWithFlareSolver(url);
 	}
+	if (body.trimStart().startsWith("<")) {
+		throw new Error(
+			`Source ${url} is behind a Cloudflare challenge that could not be bypassed. Start FlareSolverr and check its logs.`,
+		);
+	}
 	let list: SourceFurni[];
 	try {
 		const json = JSON.parse(body);