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>

openspec/changes/simplify-frontend-add-billing/specs/backend-api/spec.md

@@ -0,0 +1,22 @@
+# Spec Delta: backend-api
+
+## ADDED Requirements
+
+### Requirement: Translation Statistics Endpoint
+The system SHALL provide a new admin API endpoint for translation usage statistics across all users.
+
+#### Scenario: Admin requests translation statistics
+- GIVEN the admin is authenticated
+- WHEN GET /api/v2/admin/translation-stats is called
+- THEN the response contains:
+  - total_translations: number of translation jobs
+  - total_input_tokens: sum of input tokens used
+  - total_output_tokens: sum of output tokens used
+  - estimated_cost: calculated cost based on token pricing
+  - by_language: breakdown of translations by target language
+  - recent_translations: list of recent translation activities
+
+#### Scenario: Non-admin user requests translation statistics
+- GIVEN a regular user is authenticated
+- WHEN GET /api/v2/admin/translation-stats is called
+- THEN the response is 403 Forbidden