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/models/user_feedback.py

@@ -0,0 +1,43 @@
+from __future__ import annotations
+
+import uuid
+
+from sqlalchemy import Index, String, Text, text
+from sqlalchemy.dialects.postgresql import JSONB, UUID
+from sqlalchemy.orm import Mapped, mapped_column
+
+from core.db.base import Base, TimestampMixin
+
+
+class UserFeedback(TimestampMixin, Base):
+    __tablename__ = "user_feedback"
+    __table_args__ = (
+        Index("ix_user_feedback_user_id", "user_id"),
+        Index("ix_user_feedback_created_at", "created_at"),
+        Index("ix_user_feedback_status", "status"),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        server_default=text("gen_random_uuid()"),
+        primary_key=True,
+    )
+    user_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        nullable=True,
+    )
+    feedback_type: Mapped[str] = mapped_column(
+        String(20), nullable=False, server_default="other"
+    )
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    images: Mapped[list[str]] = mapped_column(
+        JSONB, nullable=False, server_default=text("'[]'::jsonb"), default=list
+    )
+    device_info: Mapped[dict] = mapped_column(
+        JSONB, nullable=False, server_default=text("'{}'::jsonb"), default=dict
+    )
+    app_version: Mapped[str] = mapped_column(String(20), nullable=False)
+    os_version: Mapped[str] = mapped_column(String(50), nullable=False)
+    status: Mapped[str] = mapped_column(
+        String(20), nullable=False, server_default="pending"
+    )