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/models/__init__.py

@@ -9,6 +9,7 @@ from app.models.user import User
 from app.models.task import Task, TaskFile, TaskStatus
 from app.models.session import Session
 from app.models.audit_log import AuditLog
+from app.models.translation_log import TranslationLog
 
 __all__ = [
     "User",
@@ -17,4 +18,5 @@ __all__ = [
     "TaskStatus",
     "Session",
     "AuditLog",
+    "TranslationLog",
 ]