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.

src/registry.rs

@@ -21,7 +21,10 @@ impl Registry {
                 .as_ref()
                 .and_then(|name| secrets.resolve(name));
             let provider: Arc<dyn Provider> = match m.provider.as_str() {
-                "openai" | "ollama" | "http" => Arc::new(OpenAiProvider::new(m, api_key)?),
+                // openrouter + lmstudio are OpenAI-compatible (Bearer + /chat/completions)
+                "openai" | "ollama" | "http" | "openrouter" | "lmstudio" => {
+                    Arc::new(OpenAiProvider::new(m, api_key)?)
+                }
                 "anthropic" => Arc::new(AnthropicProvider::new(m, api_key)?),
                 other => anyhow::bail!("unknown provider '{}' for model '{}'", other, m.id),
             };