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/main.go

@@ -17,6 +17,8 @@ var (
 		ListenPort: getEnvIntOrDefault("PORT", 8080),
 		DataDir:    getEnvOrDefault("DATA_DIR", "./data"),
 		StaticDir:  getEnvOrDefault("STATIC_DIR", ""),
+		LLMEndpoint: getEnvOrDefault("LLM_ENDPOINT", ""),
+		LLMKey:     getEnvOrDefault("LLM_KEY", ""),
 	}
 	rootCmd = &cobra.Command{Use: "aethera"}
 )
@@ -26,6 +28,8 @@ func init() {
 	rootCmd.PersistentFlags().StringVar(&params.StaticDir, "static-dir", params.StaticDir, "Directory to serve static frontend files from instead of embedded assets (env: AETHERA_STATIC_DIR)")
 	rootCmd.PersistentFlags().StringVar(&params.ListenAddr, "listen", params.ListenAddr, "Address to listen on (env: AETHERA_LISTEN)")
 	rootCmd.PersistentFlags().IntVar(&params.ListenPort, "port", params.ListenPort, "Port to listen on (env: AETHERA_PORT)")
+	rootCmd.PersistentFlags().StringVar(&params.LLMEndpoint, "llm-endpoint", params.LLMEndpoint, "LLM API endpoint URL (env: AETHERA_LLM_ENDPOINT)")
+	rootCmd.PersistentFlags().StringVar(&params.LLMKey, "llm-key", params.LLMKey, "LLM API key (env: AETHERA_LLM_KEY)")
 }
 
 func main() {
@@ -42,7 +46,7 @@ func main() {
 
 	// Start Server
 	rootCmd.Run = func(cmd *cobra.Command, args []string) {
-		server.StartServer(fileStore, params.DataDir, params.StaticDir, params.ListenAddr, params.ListenPort)
+		server.StartServer(fileStore, params.DataDir, params.StaticDir, params.ListenAddr, params.ListenPort, params.LLMEndpoint, params.LLMKey)
 	}
 
 	if err := rootCmd.Execute(); err != nil {