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>

blueprints/users/models.py

@@ -12,6 +12,10 @@ class User(Model):
     email = fields.CharField(max_length=100, unique=True)
     whatsapp_number = fields.CharField(max_length=30, unique=True, null=True, index=True)
 
+    # Email channel fields
+    email_enabled = fields.BooleanField(default=False)
+    email_hmac_token = fields.CharField(max_length=16, unique=True, null=True, index=True)
+
     # OIDC fields
     oidc_subject = fields.CharField(
         max_length=255, unique=True, null=True, index=True