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/api/documents.py

@@ -63,9 +63,52 @@ class DocumentUpdate(BaseModel):
     data_origin: str | None = None
 
 
+# ─── 스키마 (트리) ───
+
+
+class SubGroupNode(BaseModel):
+    sub_group: str
+    count: int
+
+
+class DomainNode(BaseModel):
+    domain: str
+    count: int
+    children: list[SubGroupNode]
+
+
 # ─── 엔드포인트 ───
 
 
+@router.get("/tree", response_model=list[DomainNode])
+async def get_document_tree(
+    user: Annotated[User, Depends(get_current_user)],
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """도메인/sub_group 트리 (사이드바용)"""
+    from sqlalchemy import text as sql_text
+
+    result = await session.execute(
+        sql_text("""
+            SELECT ai_domain, ai_sub_group, COUNT(*)
+            FROM documents
+            WHERE ai_domain IS NOT NULL
+            GROUP BY ai_domain, ai_sub_group
+            ORDER BY ai_domain, ai_sub_group
+        """)
+    )
+
+    tree: dict[str, DomainNode] = {}
+    for domain, sub_group, count in result:
+        if domain not in tree:
+            tree[domain] = DomainNode(domain=domain, count=0, children=[])
+        tree[domain].count += count
+        if sub_group:
+            tree[domain].children.append(SubGroupNode(sub_group=sub_group, count=count))
+
+    return list(tree.values())
+
+
 @router.get("/", response_model=DocumentListResponse)
 async def list_documents(
     user: Annotated[User, Depends(get_current_user)],