refactor: complete V1 to V2 migration and remove legacy architecture

Remove all V1 architecture components and promote V2 to primary:
- Delete all paddle_ocr_* table models (export, ocr, translation, user)
- Delete legacy routers (auth, export, ocr, translation)
- Delete legacy schemas and services
- Promote user_v2.py to user.py as primary user model
- Update all imports and dependencies to use V2 models only
- Update main.py version to 2.0.0

Database changes:
- Fix SQLAlchemy reserved word: rename audit_log.metadata to extra_data
- Add migration to drop all paddle_ocr_* tables
- Update alembic env to only import V2 models

Frontend fixes:
- Fix Select component exports in TaskHistoryPage.tsx
- Update to use simplified Select API with options prop
- Fix AxiosInstance TypeScript import syntax

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

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

backend/app/models/audit_log.py

@@ -67,7 +67,7 @@ class AuditLog(Base):
         comment="1 for success, 0 for failure"
     )
     error_message = Column(Text, nullable=True, comment="Error details if failed")
-    metadata = Column(Text, nullable=True, comment="Additional JSON metadata")
+    extra_data = Column(Text, nullable=True, comment="Additional JSON metadata")
     created_at = Column(DateTime, default=datetime.utcnow, nullable=False, index=True)
 
     # Relationships
@@ -90,6 +90,6 @@ class AuditLog(Base):
             "resource_id": self.resource_id,
             "success": bool(self.success),
             "error_message": self.error_message,
-            "metadata": self.metadata,
+            "extra_data": self.extra_data,
             "created_at": self.created_at.isoformat() if self.created_at else None
         }