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

docs/protocols/common/http-error-codes.md

@@ -89,6 +89,17 @@ This document is the source of truth for backend RFC7807 `code` values consumed
 |---|---:|---|---|
 | `INVITE_CODE_NOT_FOUND` | 404 | Invite code not found for current user | Show not-found message and trigger invite code bootstrap |
 
+## Feedback
+
+| code | status | meaning | frontend handling |
+|---|---:|---|---|
+| `FEEDBACK_CONTENT_EMPTY` | 400 | Feedback content is empty | Prompt user to enter content |
+| `FEEDBACK_CONTENT_TOO_LONG` | 400 | Feedback content exceeds 500 characters | Show character limit hint |
+| `FEEDBACK_TOO_MANY_IMAGES` | 400 | More than 3 images uploaded | Show image count limit hint |
+| `FEEDBACK_IMAGE_TOO_LARGE` | 400 | Image file exceeds 5 MB | Show image size limit hint |
+| `FEEDBACK_INVALID_IMAGE_TYPE` | 400 | Image type not supported (only jpg/png) | Show supported format hint |
+| `FEEDBACK_SUBMIT_FAILED` | 500 | Feedback submission failed | Show retry prompt |
+
 ## Global
 
 | code | status | meaning | frontend handling |