fix: use Google Cloud API key for classic mode + translation verification

Two critical fixes:

1. Provider "google" (default classic mode) now checks for a Google Cloud
   API key (GOOGLE_CLOUD_API_KEY in env or admin settings). If present,
   uses GoogleCloudTranslationProvider (official API). Previously it
   always fell through to deep_translator (free scraper) which gets
   blocked in production, silently returning untranslated text.

2. Added translation verification: each translator now tracks how many
   texts were attempted vs actually changed. If 0 texts were translated,
   the job is marked as FAILED with a clear error message instead of
   returning the original file as "completed".

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

routes/translate_routes.py

@@ -951,7 +951,21 @@ async def _run_translation_job(
         translation_provider = None
         _p = provider.lower()
 
-        if _p in ("openrouter", "llm") and api_key:
+        # "google" (default classic mode): use Google Cloud API key if available,
+        # otherwise fall back to deep_translator (legacy, no key).
+        if _p == "google":
+            gc_key = _cfg(
+                getattr(_admin_cfg.google_cloud, "api_key", None),
+                "GOOGLE_CLOUD_API_KEY",
+            )
+            if gc_key:
+                from services.providers.google_cloud_provider import LegacyGoogleCloudAdapter
+                translation_provider = LegacyGoogleCloudAdapter(gc_key)
+                logger.info("google_provider_using_cloud_api", job_id=job_id)
+            else:
+                logger.info("google_provider_no_cloud_key_using_legacy", job_id=job_id)
+
+        elif _p in ("openrouter", "llm") and api_key:
             translation_provider = OpenRouterTranslationProvider(
                 api_key, model, full_prompt
             )
@@ -1114,6 +1128,30 @@ async def _run_translation_job(
         else:
             raise ValueError(f"Unsupported file type: {file_extension}")
 
+        # ── Verify translation actually produced results ──
+        if not output_path.exists() or output_path.stat().st_size == 0:
+            error_msg = "Translation failed: output file is empty or missing. The translation provider may be unavailable."
+            logger.error(f"Job {job_id}: {error_msg}")
+            tracker.set_error(error_msg)
+            return
+
+        stats = job_translator.get_translation_stats()
+        attempted = stats.get("attempted", 0)
+        changed = stats.get("changed", 0)
+
+        if attempted > 0:
+            ratio = changed / attempted
+            logger.info(f"Job {job_id}: translation stats — {changed}/{attempted} texts changed ({ratio:.0%})")
+            if ratio < 0.15 and changed == 0:
+                error_msg = (
+                    f"Translation failed: 0 out of {attempted} texts were translated. "
+                    f"The provider ({provider}) may be unavailable or misconfigured. "
+                    f"Check your API keys in admin settings."
+                )
+                logger.error(f"Job {job_id}: {error_msg}")
+                tracker.set_error(error_msg)
+                return
+
         if user_id:
             await tier_quota_service.increment_on_success(user_id)
             # Persist monthly usage counters in PostgreSQL (docs + pages)