feat: add data backup/import, goal step ordering, and PostgreSQL migration

- Add GET /api/backup/export and POST /api/backup/import endpoints for full data backup
- Add drag-and-drop reorder for goal steps with PUT /api/goals/{id}/steps/reorder
- Auto-assign sort_order on step creation (preserves creation order)
- Fix duplicate milestone rendering in goal detail page
- Add category management button in goal dialog
- Migrate database default from SQLite to PostgreSQL
- Fix router guard redirect loop for logged-in users on setup/login pages
- Fix ALTER TABLE ADD COLUMN crash on callable defaults (uuid.uuid4)
- Add auth status rate limiter and token version caching
- Update CLAUDE.md to reflect current architecture

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/schemas/backup.py

@@ -0,0 +1,14 @@
+"""数据备份导入导出 Schema"""
+from pydantic import BaseModel
+from datetime import datetime, date
+from typing import Optional, Any
+
+
+class BackupMetadata(BaseModel):
+    version: int = 1
+    exported_at: datetime
+
+
+class BackupPayload(BaseModel):
+    metadata: BackupMetadata
+    data: dict[str, list[dict[str, Any]]]