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/email/helpers.py

@@ -0,0 +1,14 @@
+import hmac
+import hashlib
+
+
+def generate_email_token(user_id: str, secret: str) -> str:
+    """Generate a 16-char hex HMAC token for a user's email address."""
+    return hmac.new(
+        secret.encode(), str(user_id).encode(), hashlib.sha256
+    ).hexdigest()[:16]
+
+
+def get_user_email_address(token: str, domain: str) -> str:
+    """Return the routable email address for a given token."""
+    return f"ask+{token}@{domain}"