fix(httpclient): fix HTTP 0 status and restore FS session default

- Guard isCloudflareChallenge with directStatus >= 400 to prevent
  overriding status to 0 when no direct request was made
- When FS returns challenge page without a prior direct status,
  return an error instead of silently passing HTTP 0
- Restore default FS session ID to 'goyomi' — without a session,
  each request spawns a new Chrome, causing timeouts under load
- Add Message field to FlareSolverrResponse for better error reporting
- Document FLARESOLVERR_SESSION env var: shared session = fast after
  1st request, but serializes. Set empty for parallel (resource-heavy).

internal/httpclient/client.go

@@ -211,12 +211,13 @@ func (c *Client) doFS(req *http.Request, directStatus int) (*http.Response, erro
 		}
 	}
 
-	// When FlareSolverr returns status 200, Chrome rendered the page.
-	// Check if the body actually contains Cloudflare challenge indicators
-	// rather than relying on structural heuristics (<pre> wrapper).
-	if statusCode == 200 {
-		if isCloudflareChallenge([]byte(rawBody)) {
+	// If FS returned the challenge page instead of the real content,
+	// reject it (HTTP 0 case when directStatus=0).
+	if statusCode == 200 && isCloudflareChallenge([]byte(rawBody)) {
+		if directStatus >= 400 {
 			statusCode = directStatus
+		} else {
+			return nil, fmt.Errorf("FlareSolverr returned challenge page for %s", rawURL)
 		}
 	}