fix: fallback to free Google Translate when Cloud API key is invalid/blocked

Google Cloud Translation API was returning "Requests to this API are
blocked" which got wrapped as a misleading "Erreur lors de la lecture
du fichier PowerPoint". Now probes the key once (cached 10min) and
falls back to deep_translator (free) when the Cloud key is invalid.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

routes/translate_routes.py

@@ -368,6 +368,35 @@ async def download_from_url(url: str, timeout: int = 30) -> tuple[Path, str]:
 _translation_jobs: dict[str, dict] = {}
 _JOB_TTL_SECONDS = 3600
 _last_cleanup_ts: float = 0.0
+
+# Google Cloud API key validity cache — avoids probing the API on every request.
+_gc_key_cache: dict[str, tuple[bool, float]] = {}
+_GC_KEY_CACHE_TTL = 600  # 10 minutes
+
+
+def _google_cloud_key_valid(api_key: str, job_id: str) -> bool:
+    """Check if a Google Cloud API key is valid, with a 10-minute cache."""
+    import time
+    now = time.time()
+    cached = _gc_key_cache.get(api_key)
+    if cached:
+        is_valid, ts = cached
+        if now - ts < _GC_KEY_CACHE_TTL:
+            return is_valid
+    # Probe the API with a tiny translation
+    try:
+        from services.providers.google_cloud_provider import LegacyGoogleCloudAdapter
+        _test = LegacyGoogleCloudAdapter(api_key)
+        _test.translate("test", "fr", "en")
+        _gc_key_cache[api_key] = (True, now)
+        return True
+    except Exception as _gc_err:
+        logger.warning(
+            "google_cloud_key_invalid",
+            extra={"job_id": job_id, "error": str(_gc_err)[:200]},
+        )
+        _gc_key_cache[api_key] = (False, now)
+        return False
 _CLEANUP_INTERVAL_SECONDS = 300  # run cleanup every 5 minutes at most
 
 
@@ -999,8 +1028,8 @@ async def _run_translation_job(
         _p = provider.lower()
 
         # "google" (default classic mode): use Google Cloud API key if available.
-        # We no longer fall back silently to the free legacy translator if the Cloud API fails.
-        # If the key is invalid or quota is exceeded, we want the error to bubble up to the user.
+        # If the Cloud API key is invalid or the API is not enabled, fall back
+        # to the free legacy Google Translate (deep_translator) instead of failing.
         if _p == "google":
             # the user might have set GOOGLE_API_KEY instead of GOOGLE_CLOUD_API_KEY
             gc_key = _cfg(
@@ -1008,14 +1037,14 @@ async def _run_translation_job(
                 "GOOGLE_CLOUD_API_KEY",
             ) or os.getenv("GOOGLE_API_KEY", "").strip()
 
-            if gc_key:
+            if gc_key and _google_cloud_key_valid(gc_key, job_id):
                 from services.providers.google_cloud_provider import LegacyGoogleCloudAdapter
                 translation_provider = LegacyGoogleCloudAdapter(gc_key)
                 logger.info("google_provider_using_cloud_api", extra={"job_id": job_id})
             else:
                 from services.translation_service import GoogleTranslationProvider
                 translation_provider = GoogleTranslationProvider()
-                logger.info("google_provider_no_cloud_key_using_legacy", extra={"job_id": job_id})
+                logger.info("google_provider_using_legacy", extra={"job_id": job_id})
 
         elif _p in ("openrouter", "llm") and api_key:
             translation_provider = OpenRouterTranslationProvider(