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>

.env.example

@@ -70,6 +70,17 @@ TWILIO_SIGNATURE_VALIDATION=true
 # WHATSAPP_RATE_LIMIT_MAX=10
 # WHATSAPP_RATE_LIMIT_WINDOW=60
 
+# Mailgun Configuration (Email channel)
+MAILGUN_API_KEY=
+MAILGUN_DOMAIN=
+MAILGUN_WEBHOOK_SIGNING_KEY=
+EMAIL_HMAC_SECRET=
+# Rate limiting: max emails per window (default: 5 per 300 seconds)
+# EMAIL_RATE_LIMIT_MAX=5
+# EMAIL_RATE_LIMIT_WINDOW=300
+# Set to false to disable Mailgun signature validation in development
+MAILGUN_SIGNATURE_VALIDATION=true
+
 # Obsidian Configuration (headless sync)
 # Auth token from Obsidian account (Settings → Account → API token)
 OBSIDIAN_AUTH_TOKEN=your-obsidian-auth-token