fix(server): make CORS origins configurable and drop the credentialed wildcard (#448)

[Anai-Guo]

Summary

Fixes the CORS portion of #448. TabbyAPI ships:

app.add_middleware(
    CORSMiddleware,
    allow_origins=["*"],
    allow_credentials=True,
    allow_methods=["*"],
    allow_headers=["*"],
)

Because allow_credentials=True is combined with allow_origins=["*"], Starlette reflects the requesting origin (rather than sending a literal *). Any web page a user has open in their browser is also "connecting from localhost", so it can pass preflight, POST to http://127.0.0.1:5000, and read the responses. For an instance running with disable_auth: true — which the config sample explicitly recommends for localhost-only use — that page reaches the admin surface (/v1/model/load, /v1/download, /v1/model/list with the resolved model path, ...). Thanks @elfrost for the detailed write-up and repro.

Change

• Add a network.allowed_origins config key so the CORS allowlist is configurable. Default is ["*"], preserving today's behavior so existing browser frontends keep working out of the box.
• Set allow_credentials=False. TabbyAPI authenticates with a header/query token, never cookies, so credentialed CORS provides nothing and only enables the origin-reflection footgun. With this change the middleware sends a literal * (or the configured allowlist) instead of reflecting an arbitrary origin, and operators who want to lock down a shared instance can now set an explicit allowlist or an empty list [].

network:
  # Restrict to your own frontends, or [] to block all cross-origin callers.
  allowed_origins: ["http://localhost:8000"]

Note on the default

I kept the default permissive (["*"]) to avoid breaking existing browser UIs on upgrade, and scoped this PR to exposing the knob + removing the credentialed-wildcard footgun. If you'd prefer a secure-by-default posture (default [], browser callers opt in), that's a one-line change to the field default — happy to flip it if you want that instead.

The second item in #448 (the image_url server-side fetch / SSRF surface) is orthogonal and left for a separate change.

Testing

config_sample.yml is updated to document the new key. The repo's tests/ are live-server integration scripts (they need a loaded model), so there's no unit harness for middleware wiring to extend here; the change is config-driven and common/config_models.py + endpoints/server.py compile clean.

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