feat(feedback): implement user feedback collection system with email reporting

Backend:
- Add user_feedback table with RLS policy
- Create feedback submission API (multipart/form-data)
- Implement xlsx report generation with embedded images
- Add scheduled email delivery via Feishu SMTP
- Create HTML email templates (daily_report, no_feedback)

Frontend:
- Add feedback screen with type selection and image picker
- Support anonymous submission via skipAuth flag
- Collect device info and app version

Protocol:
- Document feedback API contract and error codes
- Update http-error-codes.md with FEEDBACK_* codes

backend/src/v1/feedback/repository.py

@@ -0,0 +1,40 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from uuid import UUID
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from models.user_feedback import UserFeedback
+
+
+@dataclass
+class FeedbackRepository:
+    session: AsyncSession
+
+    async def create_feedback(
+        self,
+        *,
+        user_id: UUID | None,
+        feedback_type: str,
+        content: str,
+        images: list[str],
+        device_info: dict,
+        app_version: str,
+        os_version: str,
+    ) -> UserFeedback:
+        feedback = UserFeedback(
+            user_id=user_id,
+            feedback_type=feedback_type,
+            content=content,
+            images=images,
+            device_info=device_info,
+            app_version=app_version,
+            os_version=os_version,
+        )
+        self.session.add(feedback)
+        await self.session.flush()
+        return feedback
+
+    async def save(self) -> None:
+        await self.session.commit()