feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

backend/internal/repository/github_release_service.go

@@ -98,12 +98,16 @@ func (c *githubReleaseClient) DownloadFile(ctx context.Context, url, dest string
 	if err != nil {
 		return err
 	}
-	defer func() { _ = out.Close() }()
 
 	// SECURITY: Use LimitReader to enforce max download size even if Content-Length is missing/wrong
 	limited := io.LimitReader(resp.Body, maxSize+1)
 	written, err := io.Copy(out, limited)
+
+	// Close file before attempting to remove (required on Windows)
+	_ = out.Close()
+
 	if err != nil {
+		_ = os.Remove(dest) // Clean up partial file (best-effort)
 		return err
 	}