fix(channels): harden runtime credential management APIs (#3581)

* fix(channels): harden runtime credential management APIs

* fix(channels): address review feedback on credential hardening

Follow-up to the runtime credential-hardening pass, resolving five review
findings:

- WeChat auth persistence now writes through a 0o600 NamedTemporaryFile +
  Path.replace instead of write_text-then-chmod, so the iLink bot_token is
  never briefly readable at umask defaults (mirrors ChannelRuntimeConfigStore).
- The post-write chmod is split into its own try/except: a chmod failure on a
  filesystem without POSIX perms now logs at debug instead of masquerading as
  a "failed to persist" warning.
- Extracted the three near-identical _require_admin_user helpers (mcp,
  channel_connections, channels) into a single require_admin_user(request, *,
  detail) in app/gateway/deps.py; each router supplies its own detail string.
- Strengthened the runtime-config-store chmod coverage: a new test injects a
  temp-file chmod failure and asserts it is logged at debug while the
  destination is still owner-only (mutation-verified to fail if the chmod is
  dropped), plus a loose-pre-existing-file case.
- Removed the unused _FakeRepo from the blocking-io test: its isinstance gate
  routes through the repo-less 503 path, so neither stub was ever invoked.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

backend/app/gateway/routers/channels.py

@@ -4,13 +4,17 @@ from __future__ import annotations
 
 import logging
 
-from fastapi import APIRouter, HTTPException
+from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel
 
+from app.gateway.deps import require_admin_user
+
 logger = logging.getLogger(__name__)
 
 router = APIRouter(prefix="/api/channels", tags=["channels"])
 
+_ADMIN_REQUIRED_DETAIL = "Admin privileges required to manage channel runtime workers."
+
 
 class ChannelStatusResponse(BaseModel):
     service_running: bool
@@ -35,8 +39,10 @@ async def get_channels_status() -> ChannelStatusResponse:
 
 
 @router.post("/{name}/restart", response_model=ChannelRestartResponse)
-async def restart_channel(name: str) -> ChannelRestartResponse:
+async def restart_channel(name: str, request: Request) -> ChannelRestartResponse:
     """Restart a specific IM channel."""
+    await require_admin_user(request, detail=_ADMIN_REQUIRED_DETAIL)
+
     from app.channels.service import get_channel_service
 
     service = get_channel_service()