webapi: add a Host allowlist and Origin/Sec-Fetch-Site checks to the loopback server

[alencheung]

The webapi binds loopback by default (argus_skill/webapi/server.py, serve() at L712-714: host 127.0.0.1, port 8799), and the docs note that this default "needs no token" (docs/mobile.md ~L112). Two standard loopback-server guards are currently absent:

1. No Host-header allowlist — create_app installs no TrustedHostMiddleware or equivalent, so the server answers requests regardless of the Host header they carry.
2. No Origin / Sec-Fetch-Site validation — the CORS middleware (L631-641) only governs preflights and response headers; nothing checks the Origin or Sec-Fetch-Site of the requests themselves.

Loopback services without these checks are the classic target of DNS rebinding (a page served from a rebound hostname becomes same-origin to the port) and are reachable by any other local user on multi-user machines, since a loopback bind is machine-global. The desktop host already models the stronger posture — it always sets ARGUS_SKILL_WEB_TOKEN for the web UI it launches (desktop-tauri/src-tauri/src/backend.rs ~L920).

Suggested hardening:

• Reject requests whose Host is not 127.0.0.1[:8799] / localhost[:8799].
• Require Sec-Fetch-Site: same-origin (or a matching Origin) on non-GET requests.
• Optionally, mint a per-run random token for the tokenless loopback mode, as the desktop host already does — this also covers the multi-user-machine case.