feat(auth): wire auth end-to-end (middleware + frontend replacement)

Backend:
- Port auth_middleware, csrf_middleware, langgraph_auth, routers/auth
- Port authz decorator (owner_filter_key defaults to 'owner_id')
- Merge app.py: register AuthMiddleware + CSRFMiddleware + CORS, add
  _ensure_admin_user lifespan hook, _migrate_orphaned_threads helper,
  register auth router
- Merge deps.py: add get_local_provider, get_current_user_from_request,
  get_optional_user_from_request; keep get_current_user as thin str|None
  adapter for feedback router
- langgraph.json: add auth path pointing to langgraph_auth.py:auth
- Rename metadata['user_id'] -> metadata['owner_id'] in langgraph_auth
  (both metadata write and LangGraph filter dict) + test fixtures

Frontend:
- Delete better-auth library and api catch-all route
- Remove better-auth npm dependency and env vars (BETTER_AUTH_SECRET,
  BETTER_AUTH_GITHUB_*) from env.js
- Port frontend/src/core/auth/* (AuthProvider, gateway-config,
  proxy-policy, server-side getServerSideUser, types)
- Port frontend/src/core/api/fetcher.ts
- Port (auth)/layout, (auth)/login, (auth)/setup pages
- Rewrite workspace/layout.tsx as server component that calls
  getServerSideUser and wraps in AuthProvider
- Port workspace/workspace-content.tsx for the client-side sidebar logic

Tests:
- Port 5 auth test files (test_auth, test_auth_middleware,
  test_auth_type_system, test_ensure_admin, test_langgraph_auth)
- 176 auth tests PASS

After this commit: login/logout/registration flow works, but persistence
layer does not yet filter by owner_id. Commit 4 closes that gap.

backend/app/gateway/langgraph_auth.py

@@ -0,0 +1,106 @@
+"""LangGraph Server auth handler — shares JWT logic with Gateway.
+
+Loaded by LangGraph Server via langgraph.json ``auth.path``.
+Reuses the same ``decode_token`` / ``get_auth_config`` as Gateway,
+so both modes validate tokens with the same secret and rules.
+
+Two layers:
+  1. @auth.authenticate — validates JWT cookie, extracts user_id,
+     and enforces CSRF on state-changing methods (POST/PUT/DELETE/PATCH)
+  2. @auth.on — returns metadata filter so each user only sees own threads
+"""
+
+import secrets
+
+from langgraph_sdk import Auth
+
+from app.gateway.auth.errors import TokenError
+from app.gateway.auth.jwt import decode_token
+from app.gateway.deps import get_local_provider
+
+auth = Auth()
+
+# Methods that require CSRF validation (state-changing per RFC 7231).
+_CSRF_METHODS = frozenset({"POST", "PUT", "DELETE", "PATCH"})
+
+
+def _check_csrf(request) -> None:
+    """Enforce Double Submit Cookie CSRF check for state-changing requests.
+
+    Mirrors Gateway's CSRFMiddleware logic so that LangGraph routes
+    proxied directly by nginx have the same CSRF protection.
+    """
+    method = getattr(request, "method", "") or ""
+    if method.upper() not in _CSRF_METHODS:
+        return
+
+    cookie_token = request.cookies.get("csrf_token")
+    header_token = request.headers.get("x-csrf-token")
+
+    if not cookie_token or not header_token:
+        raise Auth.exceptions.HTTPException(
+            status_code=403,
+            detail="CSRF token missing. Include X-CSRF-Token header.",
+        )
+
+    if not secrets.compare_digest(cookie_token, header_token):
+        raise Auth.exceptions.HTTPException(
+            status_code=403,
+            detail="CSRF token mismatch.",
+        )
+
+
+@auth.authenticate
+async def authenticate(request):
+    """Validate the session cookie, decode JWT, and check token_version.
+
+    Same validation chain as Gateway's get_current_user_from_request:
+      cookie → decode JWT → DB lookup → token_version match
+    Also enforces CSRF on state-changing methods.
+    """
+    # CSRF check before authentication so forged cross-site requests
+    # are rejected early, even if the cookie carries a valid JWT.
+    _check_csrf(request)
+
+    token = request.cookies.get("access_token")
+    if not token:
+        raise Auth.exceptions.HTTPException(
+            status_code=401,
+            detail="Not authenticated",
+        )
+
+    payload = decode_token(token)
+    if isinstance(payload, TokenError):
+        raise Auth.exceptions.HTTPException(
+            status_code=401,
+            detail=f"Token error: {payload.value}",
+        )
+
+    user = await get_local_provider().get_user(payload.sub)
+    if user is None:
+        raise Auth.exceptions.HTTPException(
+            status_code=401,
+            detail="User not found",
+        )
+    if user.token_version != payload.ver:
+        raise Auth.exceptions.HTTPException(
+            status_code=401,
+            detail="Token revoked (password changed)",
+        )
+
+    return payload.sub
+
+
+@auth.on
+async def add_owner_filter(ctx: Auth.types.AuthContext, value: dict):
+    """Inject owner_id metadata on writes; filter by owner_id on reads.
+
+    Gateway stores thread ownership as ``metadata.owner_id``.
+    This handler ensures LangGraph Server enforces the same isolation.
+    """
+    # On create/update: stamp owner_id into metadata
+    metadata = value.setdefault("metadata", {})
+    metadata["owner_id"] = ctx.user.identity
+
+    # Return filter dict — LangGraph applies it to search/read/delete
+    return {"owner_id": ctx.user.identity}