fix(ci): Remove buildx cache and add preflight enforcement

- Remove cache_from/cache_to from CI (plugin bug splitting commas)
- Add preflight() method to CrawlRotator - tests proxy + anti-detect
- Add pre-task preflight check - workers MUST pass before executing
- Add releaseTask() to release tasks back to pending on preflight fail
- Rename proxy_test task to whoami for clarity

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/services/crawl-rotator.ts

@@ -683,6 +683,118 @@ export class CrawlRotator {
     const current = this.proxy.getCurrent();
     return current?.timezone;
   }
+
+  /**
+   * Preflight check - verifies proxy and anti-detect are working
+   * MUST be called before any task execution to ensure anonymity.
+   *
+   * Tests:
+   * 1. Proxy available - a proxy must be loaded and active
+   * 2. Proxy connectivity - makes HTTP request through proxy to verify connection
+   * 3. Anti-detect headers - verifies fingerprint is set with required headers
+   *
+   * @returns Promise<PreflightResult> with pass/fail status and details
+   */
+  async preflight(): Promise<PreflightResult> {
+    const result: PreflightResult = {
+      passed: false,
+      proxyAvailable: false,
+      proxyConnected: false,
+      antidetectReady: false,
+      proxyIp: null,
+      fingerprint: null,
+      error: null,
+      responseTimeMs: null,
+    };
+
+    // Step 1: Check proxy is available
+    const currentProxy = this.proxy.getCurrent();
+    if (!currentProxy) {
+      result.error = 'No proxy available';
+      console.log('[Preflight] FAILED - No proxy available');
+      return result;
+    }
+    result.proxyAvailable = true;
+    result.proxyIp = currentProxy.host;
+
+    // Step 2: Check fingerprint/anti-detect is ready
+    const fingerprint = this.userAgent.getCurrent();
+    if (!fingerprint || !fingerprint.userAgent) {
+      result.error = 'Anti-detect fingerprint not initialized';
+      console.log('[Preflight] FAILED - No fingerprint');
+      return result;
+    }
+    result.antidetectReady = true;
+    result.fingerprint = {
+      userAgent: fingerprint.userAgent,
+      browserName: fingerprint.browserName,
+      deviceCategory: fingerprint.deviceCategory,
+    };
+
+    // Step 3: Test proxy connectivity with an actual HTTP request
+    // Use httpbin.org/ip to verify request goes through proxy
+    const proxyUrl = this.proxy.getProxyUrl(currentProxy);
+    const testUrl = 'https://httpbin.org/ip';
+
+    try {
+      const { default: axios } = await import('axios');
+      const { HttpsProxyAgent } = await import('https-proxy-agent');
+
+      const agent = new HttpsProxyAgent(proxyUrl);
+      const startTime = Date.now();
+
+      const response = await axios.get(testUrl, {
+        httpsAgent: agent,
+        timeout: 15000, // 15 second timeout
+        headers: {
+          'User-Agent': fingerprint.userAgent,
+          'Accept-Language': fingerprint.acceptLanguage,
+          ...(fingerprint.secChUa && { 'sec-ch-ua': fingerprint.secChUa }),
+          ...(fingerprint.secChUaPlatform && { 'sec-ch-ua-platform': fingerprint.secChUaPlatform }),
+          ...(fingerprint.secChUaMobile && { 'sec-ch-ua-mobile': fingerprint.secChUaMobile }),
+        },
+      });
+
+      result.responseTimeMs = Date.now() - startTime;
+      result.proxyConnected = true;
+      result.passed = true;
+
+      // Mark success on proxy stats
+      await this.proxy.markSuccess(currentProxy.id, result.responseTimeMs);
+
+      console.log(`[Preflight] PASSED - Proxy ${currentProxy.host} connected (${result.responseTimeMs}ms), UA: ${fingerprint.browserName}/${fingerprint.deviceCategory}`);
+    } catch (err: any) {
+      result.error = `Proxy connection failed: ${err.message || 'Unknown error'}`;
+      console.log(`[Preflight] FAILED - Proxy connection error: ${err.message}`);
+
+      // Mark failure on proxy stats
+      await this.proxy.markFailed(currentProxy.id, err.message);
+    }
+
+    return result;
+  }
+}
+
+/**
+ * Result from preflight check
+ */
+export interface PreflightResult {
+  /** Overall pass/fail */
+  passed: boolean;
+  /** Step 1: Is a proxy loaded? */
+  proxyAvailable: boolean;
+  /** Step 2: Did HTTP request through proxy succeed? */
+  proxyConnected: boolean;
+  /** Step 3: Is fingerprint/anti-detect ready? */
+  antidetectReady: boolean;
+  /** Current proxy IP */
+  proxyIp: string | null;
+  /** Fingerprint summary */
+  fingerprint: { userAgent: string; browserName: string; deviceCategory: string } | null;
+  /** Error message if failed */
+  error: string | null;
+  /** Proxy response time in ms */
+  responseTimeMs: number | null;
 }
 
 // ============================================================