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/cmd/config.go

@@ -15,6 +15,8 @@ type cliParams struct {
 	DataDir      string
 	StaticDir    string
 	SettingsFile string
+	LLMEndpoint  string
+	LLMKey       string
 }
 
 // getEnvOrDefault returns the value of an environment variable or a default value
@@ -36,6 +38,11 @@ func getEnvIntOrDefault(key string, defaultValue int) int {
 }
 
 func (p *cliParams) Validate() error {
+	// Require LLM Configuration
+	if p.LLMEndpoint == "" {
+		return fmt.Errorf("LLM endpoint is required (set AETHERA_LLM_ENDPOINT)")
+	}
+
 	// Ensure Generated Directories
 	imgDir := path.Join(p.DataDir, "generated/images")
 	if err := os.MkdirAll(imgDir, 0755); err != nil {