feat: add messaging system database models and core services
- Add Conversation, ConversationParticipant, Message, MessageAttachment models
- Add ConversationType enum (admin_vendor, vendor_customer, admin_customer)
- Add ParticipantType enum (admin, vendor, customer)
- Add Alembic migration for messaging tables
- Add MessagingService for conversation/message operations
- Add MessageAttachmentService for file upload handling
- Add message-related exceptions (ConversationNotFoundException, etc.)
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Samir Boulahtit
8b7d2fe312