feat: implement Phase 4 SvelteKit frontend + backend enhancements

Backend:
- Add dashboard API (today stats, inbox count, law alerts, pipeline status)
- Add /api/documents/tree endpoint for sidebar domain/sub_group tree
- Migrate auth to HttpOnly cookie for refresh token (XSS defense)
- Add /api/auth/logout endpoint (cookie cleanup)
- Register dashboard router in main.py

Frontend (SvelteKit + Tailwind CSS v4):
- api.ts: fetch wrapper with refresh queue pattern, 401 single retry,
  forced logout on refresh failure
- Auth store: login/logout/refresh with memory-based access token
- UI store: toast system, sidebar state
- Login page with TOTP support
- Dashboard with 4 stat widgets + recent documents
- Document list with hybrid search (debounce, URL query state, mode select)
- Document detail with format-aware viewer (markdown/PDF/HWP/Synology/fallback)
- Metadata panel (AI summary, tags, processing history)
- Inbox triage UI (batch select, confirm dialog, domain override)
- Settings page (password change, TOTP status)

Infrastructure:
- Enable frontend service in docker-compose
- Caddy path routing (/api/* → fastapi, / → frontend) + gzip

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/main.py

@@ -7,6 +7,7 @@ from fastapi.responses import RedirectResponse
 from sqlalchemy import func, select, text
 
 from api.auth import router as auth_router
+from api.dashboard import router as dashboard_router
 from api.documents import router as documents_router
 from api.search import router as search_router
 from api.setup import router as setup_router
@@ -61,9 +62,10 @@ app.include_router(auth_router, prefix="/api/auth", tags=["auth"])
 app.include_router(documents_router, prefix="/api/documents", tags=["documents"])
 app.include_router(search_router, prefix="/api/search", tags=["search"])
 
-# TODO: Phase 3~4에서 추가
+app.include_router(dashboard_router, prefix="/api/dashboard", tags=["dashboard"])
+
+# TODO: Phase 5에서 추가
 # app.include_router(tasks.router, prefix="/api/tasks", tags=["tasks"])
-# app.include_router(dashboard.router, prefix="/api/dashboard", tags=["dashboard"])
 # app.include_router(export.router, prefix="/api/export", tags=["export"])