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>

app.py

@@ -11,6 +11,7 @@ import blueprints.conversation.logic
 import blueprints.rag
 import blueprints.users
 import blueprints.whatsapp
+import blueprints.email
 import blueprints.users.models
 from main import consult_simba_oracle
 
@@ -34,6 +35,7 @@ app.register_blueprint(blueprints.users.user_blueprint)
 app.register_blueprint(blueprints.conversation.conversation_blueprint)
 app.register_blueprint(blueprints.rag.rag_blueprint)
 app.register_blueprint(blueprints.whatsapp.whatsapp_blueprint)
+app.register_blueprint(blueprints.email.email_blueprint)
 
 
 # Database configuration with environment variable support