feat: add translation billing stats and remove Export/Settings pages

- Add TranslationLog model to track translation API usage per task
- Integrate Dify API actual price (total_price) into translation stats
- Display translation statistics in admin dashboard with per-task costs
- Remove unused Export and Settings pages to simplify frontend
- Add GET /api/v2/admin/translation-stats endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/services/translation_service.py

@@ -232,6 +232,8 @@ class TranslationService:
         self._jobs_lock = threading.Lock()
         self._total_tokens = 0
         self._total_latency = 0.0
+        self._total_price = 0.0
+        self._currency = "USD"
 
     def _load_raw_ocr_regions(
         self,
@@ -459,6 +461,9 @@ class TranslationService:
 
             self._total_tokens += response.total_tokens
             self._total_latency += response.latency
+            self._total_price += response.total_price
+            if response.currency:
+                self._currency = response.currency
 
             # Map translations back to items
             translated_items = []
@@ -524,6 +529,9 @@ class TranslationService:
 
             self._total_tokens += response.total_tokens
             self._total_latency += response.latency
+            self._total_price += response.total_price
+            if response.currency:
+                self._currency = response.currency
 
             return TranslatedItem(
                 element_id=item.element_id,
@@ -555,7 +563,9 @@ class TranslationService:
         total_elements: int,
         processing_time: float,
         batch_count: int,
-        processing_track: str = 'direct'
+        processing_track: str = 'direct',
+        total_price: float = 0.0,
+        currency: str = 'USD'
     ) -> Dict:
         """
         Build the translation result JSON structure.
@@ -613,6 +623,8 @@ class TranslationService:
                     'total_characters': total_chars,
                     'processing_time_seconds': round(processing_time, 2),
                     'total_tokens': self._total_tokens,
+                    'total_price': round(total_price, 6),
+                    'currency': currency,
                     'batch_count': batch_count
                 },
                 'translations': {},  # Empty for OCR Track
@@ -656,6 +668,8 @@ class TranslationService:
                     'total_characters': total_chars,
                     'processing_time_seconds': round(processing_time, 2),
                     'total_tokens': self._total_tokens,
+                    'total_price': round(total_price, 6),
+                    'currency': currency,
                     'batch_count': batch_count
                 },
                 'translations': translations
@@ -687,6 +701,8 @@ class TranslationService:
         start_time = time.time()
         self._total_tokens = 0
         self._total_latency = 0.0
+        self._total_price = 0.0
+        self._currency = "USD"
 
         logger.info(
             f"Starting translation: task_id={task_id}, target={target_lang}"
@@ -752,7 +768,9 @@ class TranslationService:
                 total_elements=total_elements,
                 processing_time=processing_time,
                 batch_count=len(batches),
-                processing_track=processing_track
+                processing_track=processing_track,
+                total_price=self._total_price,
+                currency=self._currency
             )
 
             # Save result