Add email channel via Mailgun for Ask Simba

Users can now receive a unique email address (ask+<token>@domain) and
interact with Simba by sending emails. Inbound emails hit a Mailgun
webhook, are authenticated via HMAC token lookup, processed through the
LangChain agent, and replied to via the Mailgun API.

- Extract shared SIMBA_SYSTEM_PROMPT to blueprints/conversation/prompts.py
- Add email_enabled and email_hmac_token fields to User model
- Create blueprints/email with webhook, signature validation, rate limiting
- Add admin endpoints to enable/disable email per user
- Update AdminPanel with Email column, toggle, and copy-address button
- Add Mailgun env vars to .env.example
- Include database migration for new fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

raggr-frontend/src/api/userService.ts

@@ -167,6 +167,23 @@ class UserService {
     );
     if (!response.ok) throw new Error("Failed to unlink WhatsApp number");
   }
+
+  async adminToggleEmail(userId: string): Promise<AdminUserRecord> {
+    const response = await this.fetchWithRefreshToken(
+      `${this.baseUrl}/admin/users/${userId}/email`,
+      { method: "PUT" },
+    );
+    if (!response.ok) throw new Error("Failed to enable email");
+    return response.json();
+  }
+
+  async adminDisableEmail(userId: string): Promise<void> {
+    const response = await this.fetchWithRefreshToken(
+      `${this.baseUrl}/admin/users/${userId}/email`,
+      { method: "DELETE" },
+    );
+    if (!response.ok) throw new Error("Failed to disable email");
+  }
 }
 
 export interface AdminUserRecord {
@@ -175,6 +192,8 @@ export interface AdminUserRecord {
   email: string;
   whatsapp_number: string | null;
   auth_provider: string;
+  email_enabled: boolean;
+  email_address: string | null;
 }
 
 export { UserService };