Add Gmail debug email functionality

- GmailService: add get_or_create_label() and inject_debug_email() methods
- providers.py: add POST /providers/gmail/debug-email endpoint
- api.ts: add GmailDebugEmailResponse interface and gmailApi.sendDebugEmail()
- settings/page.tsx: add Send Debug Email button with success/error feedback
- Update CHANGELOG.md and docs/TODO.md

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/c609ede7-b9d7-4ab1-acdb-8d29164066cc

frontend/src/lib/api.ts

@@ -126,6 +126,13 @@ export interface GmailCredential {
   updated_at: string;
 }
 
+export interface GmailDebugEmailResponse {
+  message: string;
+  message_id: string | null;
+  thread_id: string | null;
+  label_ids: string[];
+}
+
 export interface UserSmtpConfig {
   id: number;
   user_id: number;
@@ -303,6 +310,12 @@ export const gmailApi = {
   async disconnect(): Promise<void> {
     await api.delete('/providers/gmail-credential');
   },
+
+  /** Inject a debug test email into the user's Gmail inbox. */
+  async sendDebugEmail(): Promise<GmailDebugEmailResponse> {
+    const response = await api.post<GmailDebugEmailResponse>('/providers/gmail/debug-email');
+    return response.data;
+  },
 };
 
 // ── SMTP Config API ─────────────────────────────────────────────────────