feat(models): friendlier add-model form — OpenRouter/LM Studio, direct key, test & load

- Providers: add openrouter (https://openrouter.ai/api/v1) and lmstudio
  (http://localhost:1234/v1), both OpenAI-compatible. Selecting a provider now
  auto-fills its verified base URL, cost tier and default capabilities.
- API key is typed directly in the form and stored securely as a secret under the
  provider's env name — no manual env var to manage.
- New POST /admin/api/provider/models validates the key and lists the provider's
  models; a "Test & load models" button fills a searchable dropdown. The model
  field now sits below the key (some providers list models from the key).
- Fuller capability set: tools, vision, audio, reasoning, json, caching.

web/js/i18n/en.js

@@ -129,6 +129,17 @@ export default {
   "models.form.secretMissing": "not set",
   "models.form.secretPlaceholder": "set or replace key (write-only)",
   "models.deleteConfirm": "Delete model \"{id}\"?",
+  "models.form.apiKey": "API Key",
+  "models.form.apiKeyPlaceholder": "paste your API key",
+  "models.form.apiKeyNotNeeded": "not required (local server)",
+  "models.form.apiKeyHint": "Stored securely on the gateway; it is not shown again.",
+  "models.form.keyStored": "A key is saved — leave blank to keep it, or paste a new one.",
+  "models.form.keyFromEnv": "Using a key from an environment variable.",
+  "models.form.testKey": "Test & load models",
+  "models.form.testing": "checking…",
+  "models.form.foundModels": "{n} models loaded",
+  "models.form.modelPlaceholder": "type to search…",
+  "models.form.modelHint": "Click “Test & load models” to pull the list from your key, then pick one.",
   "models.managed": "managed",
   "models.managedHint": "Provided by the managed CMF server (Settings → Local models). Configure it there.",