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/routers/__init__.py

@@ -1,5 +1,5 @@
 from fastapi import APIRouter
-from app.routers import tasks, categories, tags, user_settings, habits, anniversaries, auth, goals, sync
+from app.routers import tasks, categories, tags, user_settings, habits, anniversaries, auth, goals, sync, backup
 
 api_router = APIRouter()
 
@@ -12,3 +12,4 @@ api_router.include_router(habits.router)
 api_router.include_router(anniversaries.router)
 api_router.include_router(goals.router)
 api_router.include_router(sync.router)
+api_router.include_router(backup.router)