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/core/config.py

@@ -458,6 +458,14 @@ class Settings(BaseSettings):
     dify_max_batch_chars: int = Field(default=5000)  # Max characters per batch
     dify_max_batch_items: int = Field(default=20)  # Max items per batch
 
+    # Translation cost calculation (USD per 1M tokens) - FALLBACK only
+    # Dify API returns actual price (total_price), this is only used as fallback
+    # when actual price is not available
+    translation_cost_per_million_tokens: float = Field(
+        default=3.0,
+        description="Fallback cost per 1M tokens when Dify doesn't return actual price"
+    )
+
     # ===== Background Tasks Configuration =====
     task_queue_type: str = Field(default="memory")
     redis_url: str = Field(default="redis://localhost:6379/0")