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/server/server.go

@@ -13,12 +13,12 @@ import (
 	"reichard.io/aethera/web"
 )
 
-func StartServer(settingsStore store.Store, dataDir, staticDir, listenAddress string, listenPort int) {
+func StartServer(settingsStore store.Store, dataDir, staticDir, listenAddress string, listenPort int, llmEndpoint, llmKey string) {
 	mux := http.NewServeMux()
 
 	// Create API Instance - use settingsStore as the unified store for both settings and chat
 	logger := logrus.New()
-	api := api.New(settingsStore, dataDir, logger)
+	api := api.New(settingsStore, dataDir, logger, llmEndpoint, llmKey)
 
 	// Serve Static Assets
 	if staticDir != "" {