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/client/client_test.go

@@ -20,7 +20,7 @@ func TestSendMessage(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to parse base URL: %v", err)
 	}
-	client := NewClient(baseURL)
+	client := NewClient(baseURL, os.Getenv("AETHERA_LLM_KEY"))
 
 	// Create Context
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
@@ -69,7 +69,7 @@ func TestSummarizeChat(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to parse base URL: %v", err)
 	}
-	client := NewClient(baseURL)
+	client := NewClient(baseURL, os.Getenv("AETHERA_LLM_KEY"))
 
 	// Create Context
 	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
@@ -96,7 +96,7 @@ func TestSendMessageWithImage(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Failed to parse base URL: %v", err)
 	}
-	client := NewClient(baseURL)
+	client := NewClient(baseURL, os.Getenv("AETHERA_LLM_KEY"))
 
 	// Create Context
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)