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>

translators/pdf_translator.py

@@ -63,6 +63,7 @@ class PDFTranslator:
     def __init__(self, provider=None):
         self._provider = provider
         self._font_path: Optional[str] = None
+        self._translation_stats = {"attempted": 0, "changed": 0}
 
     def _get_font_path(self) -> Optional[str]:
         """Resolve a Unicode-capable TTF/OTF font file."""
@@ -825,18 +826,31 @@ class PDFTranslator:
         self, texts: List[str], target_language: str, source_language: str
     ) -> List[str]:
         """Translate a batch of texts."""
+        non_empty = [t for t in texts if t and t.strip()]
+        self._translation_stats["attempted"] += len(non_empty)
+
+        translated = None
         if self._provider is not None:
             try:
-                return self._provider.translate_batch(texts, target_language, source_language)
+                translated = self._provider.translate_batch(texts, target_language, source_language)
             except Exception as e:
                 logger.warning("provider_translate_failed", error=str(e))
 
-        from services.translation_service import translation_service
-        try:
-            return translation_service.translate_batch(texts, target_language, source_language)
-        except Exception as e:
-            logger.warning("legacy_translate_failed", error=str(e))
-            return texts
+        if translated is None:
+            from services.translation_service import translation_service
+            try:
+                translated = translation_service.translate_batch(texts, target_language, source_language)
+            except Exception as e:
+                logger.warning("legacy_translate_failed", error=str(e))
+                translated = texts
+
+        changed = sum(1 for orig, trans in zip(texts, translated) if orig != trans and trans.strip())
+        self._translation_stats["changed"] += changed
+
+        return translated
+
+    def get_translation_stats(self) -> dict:
+        return dict(self._translation_stats)
 
     def _validate_file(self, file_path: Path) -> None:
         if not file_path.exists():