feat(models): auto-derive id from model name; actionable probe auth errors

- The model id is now auto-filled from the model name as you type, and if left
  blank it's derived on save (slug of the last path segment, e.g.
  "qwen/qwen3.7-max" → "qwen3-7-max") — saving no longer errors on a blank id.
- probe/test auth failures (401 / "User not found" / "No auth") now append a
  hint to check the API key (for OpenRouter: use an inference key, not a
  provisioning/management key).

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cortiq-gateway"
-version = "0.2.11"
+version = "0.2.12"
 edition = "2021"
 rust-version = "1.75"
 license = "Apache-2.0"