refactor!: move LLM configuration from in-app settings to CLI/env vars

- Remove `api_endpoint` from Settings model and settings UI
- Add `--llm-endpoint` / `AETHERA_LLM_ENDPOINT` and `--llm-key` /
  `AETHERA_LLM_KEY` CLI flags (endpoint is required)
- Update client constructor to accept API key parameter
- Update tests and documentation to reflect new configuration approach

BREAKING CHANGE: LLM endpoint and key must now be provided via
`AETHERA_LLM_ENDPOINT` and `AETHERA_LLM_KEY` environment variables or
CLI flags instead of the Settings page.

backend/internal/api/handlers.go

@@ -28,14 +28,18 @@ type API struct {
 	store             store.Store
 	client            *client.Client
 	dataDir           string
+	llmEndpoint       string
+	llmKey            string
 	generationManager *generationManager
 }
 
-func New(s store.Store, dataDir string, logger *logrus.Logger) *API {
+func New(s store.Store, dataDir string, logger *logrus.Logger, llmEndpoint, llmKey string) *API {
 	return &API{
 		store:             s,
 		dataDir:           dataDir,
 		logger:            logger.WithField("service", "api"),
+		llmEndpoint:       llmEndpoint,
+		llmKey:            llmKey,
 		generationManager: newGenerationManager(),
 	}
 }
@@ -68,24 +72,6 @@ func (a *API) PostSettings(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if apiEndpoint := newSettings.APIEndpoint; apiEndpoint != "" {
-		baseURL, err := url.Parse(apiEndpoint)
-		if err != nil {
-			errMsg := fmt.Sprintf("Invalid API Endpoint URL: %q", baseURL)
-			log.WithError(err).Error(errMsg)
-			http.Error(w, errMsg, http.StatusBadRequest)
-			return
-		}
-
-		testClient := client.NewClient(baseURL)
-		if _, err := testClient.GetModels(r.Context()); err != nil {
-			log.WithError(err).Error("failed to access configured API endpoint")
-			http.Error(w, "API endpoint inaccessible", http.StatusBadRequest)
-			return
-		}
-		a.client = nil
-	}
-
 	if err := a.store.SaveSettings(&newSettings); err != nil {
 		log.WithError(err).Error("failed to save settings")
 		http.Error(w, "Failed to save application settings", http.StatusInternalServerError)
@@ -516,20 +502,13 @@ func (a *API) getClient() (*client.Client, error) {
 		return a.client, nil
 	}
 
-	// Get Settings & Validate Endpoint
-	settings, err := a.store.GetSettings()
-	if err != nil {
-		return nil, fmt.Errorf("failed to retrieve application settings: %w", err)
-	} else if settings.APIEndpoint == "" {
-		return nil, errors.New("no API endpoint configured in settings")
-	}
-
-	baseURL, err := url.Parse(settings.APIEndpoint)
+	// Parse LLM Endpoint from Config
+	baseURL, err := url.Parse(a.llmEndpoint)
 	if err != nil {
 		return nil, fmt.Errorf("invalid API endpoint URL: %w", err)
 	}
 
-	a.client = client.NewClient(baseURL)
+	a.client = client.NewClient(baseURL, a.llmKey)
 	return a.client, nil
 }