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/translation_log.py

@@ -0,0 +1,87 @@
+"""
+Tool_OCR - Translation Log Model
+Tracks translation usage statistics for billing and monitoring
+"""
+
+from sqlalchemy import Column, Integer, String, DateTime, Float, ForeignKey
+from sqlalchemy.orm import relationship
+from datetime import datetime
+
+from app.core.database import Base
+
+
+class TranslationLog(Base):
+    """
+    Translation log model for tracking API usage and costs.
+
+    Each record represents a single translation job completion,
+    storing token usage and estimated costs for billing purposes.
+    """
+
+    __tablename__ = "tool_ocr_translation_logs"
+
+    id = Column(Integer, primary_key=True, index=True, autoincrement=True)
+    user_id = Column(Integer, ForeignKey("tool_ocr_users.id", ondelete="CASCADE"),
+                    nullable=False, index=True,
+                    comment="Foreign key to users table")
+    task_id = Column(String(255), nullable=False, index=True,
+                    comment="Task UUID that was translated")
+    target_lang = Column(String(10), nullable=False, index=True,
+                        comment="Target language code (e.g., 'en', 'ja', 'zh-TW')")
+    source_lang = Column(String(10), nullable=True,
+                        comment="Source language code (or 'auto')")
+
+    # Token usage statistics
+    input_tokens = Column(Integer, default=0, nullable=False,
+                         comment="Number of input tokens used")
+    output_tokens = Column(Integer, default=0, nullable=False,
+                          comment="Number of output tokens generated")
+    total_tokens = Column(Integer, default=0, nullable=False,
+                         comment="Total tokens (input + output)")
+
+    # Translation statistics
+    total_elements = Column(Integer, default=0, nullable=False,
+                           comment="Total elements in document")
+    translated_elements = Column(Integer, default=0, nullable=False,
+                                comment="Number of elements translated")
+    total_characters = Column(Integer, default=0, nullable=False,
+                             comment="Total characters translated")
+
+    # Cost tracking (estimated based on token pricing)
+    estimated_cost = Column(Float, default=0.0, nullable=False,
+                           comment="Estimated cost in USD")
+
+    # Processing info
+    processing_time_seconds = Column(Float, default=0.0, nullable=False,
+                                    comment="Translation processing time")
+    provider = Column(String(50), default="dify", nullable=False,
+                     comment="Translation provider (e.g., 'dify')")
+
+    # Timestamps
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False, index=True)
+
+    # Relationships
+    user = relationship("User", back_populates="translation_logs")
+
+    def __repr__(self):
+        return f"<TranslationLog(id={self.id}, task_id='{self.task_id}', target_lang='{self.target_lang}', tokens={self.total_tokens})>"
+
+    def to_dict(self):
+        """Convert translation log to dictionary"""
+        return {
+            "id": self.id,
+            "user_id": self.user_id,
+            "task_id": self.task_id,
+            "target_lang": self.target_lang,
+            "source_lang": self.source_lang,
+            "input_tokens": self.input_tokens,
+            "output_tokens": self.output_tokens,
+            "total_tokens": self.total_tokens,
+            "total_elements": self.total_elements,
+            "translated_elements": self.translated_elements,
+            "total_characters": self.total_characters,
+            "estimated_cost": self.estimated_cost,
+            "processing_time_seconds": self.processing_time_seconds,
+            "provider": self.provider,
+            "created_at": self.created_at.isoformat() if self.created_at else None
+        }