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/excel_translator.py

@@ -107,6 +107,7 @@ class ExcelTranslator:
         self._provider = provider
         self.formula_pattern = re.compile(r"=.*")
         self._custom_prompt: Optional[str] = None
+        self._translation_stats = {"attempted": 0, "changed": 0}
 
     def set_provider(self, provider: TranslationProvider) -> None:
         """Set the translation provider."""
@@ -387,26 +388,26 @@ class ExcelTranslator:
     def _batch_translate(
         self, texts: List[str], target_language: str, source_language: str = "auto"
     ) -> List[str]:
-        """
-        Batch translate using new provider interface.
-
-        Args:
-            texts: List of texts to translate
-            target_language: Target language code
-            source_language: Source language code
-
-        Returns:
-            List of translated texts (same order as input)
-        """
         if not texts:
             return []
 
+        non_empty = [t for t in texts if t and t.strip()]
+        self._translation_stats["attempted"] += len(non_empty)
+
         if self._provider is not None:
-            return self._translate_with_provider(
+            translated = self._translate_with_provider(
                 texts, target_language, source_language
             )
+        else:
+            translated = self._translate_with_legacy(texts, target_language, source_language)
 
-        return self._translate_with_legacy(texts, target_language, source_language)
+        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 _translate_with_provider(
         self, texts: List[str], target_language: str, source_language: str