Add SendBlue iMessage integration with admin-only access

- New imessage blueprint: webhook receives inbound iMessages, runs through
  LangChain agent, replies via SendBlue REST API
- Admin-only: only users with lldap_admin group can use iMessage channel
- Admin endpoints to link/unlink imessage_number on user accounts
- Add imessage_number field to User model (needs aerich migration)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -93,6 +93,17 @@ EMAIL_HMAC_SECRET=
 # Set to false to disable Mailgun signature validation in development
 MAILGUN_SIGNATURE_VALIDATION=true
 
+# SendBlue Configuration (iMessage)
+SENDBLUE_API_KEY=your-sendblue-api-key
+SENDBLUE_API_SECRET=your-sendblue-api-secret
+SENDBLUE_FROM_NUMBER=+1XXXXXXXXXX
+# Comma-separated list of iMessage numbers allowed to use the service (E.164 format)
+# Use * to allow any number
+ALLOWED_IMESSAGE_NUMBERS=
+# Rate limiting: max messages per window (default: 10 messages per 60 seconds)
+# IMESSAGE_RATE_LIMIT_MAX=10
+# IMESSAGE_RATE_LIMIT_WINDOW=60
+
 # Google Calendar Configuration (via gws CLI)
 GOOGLE_CALENDAR_ENABLED=true
 # Export credentials: gws auth export --unmasked > credentials.json