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.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/schema/__init__.py

@@ -8,6 +8,7 @@ from . import (
     inventory,
     marketplace_import_job,
     marketplace_product,
+    message,
     stats,
     vendor,
 )
@@ -19,6 +20,7 @@ __all__ = [
     "base",
     "auth",
     "marketplace_product",
+    "message",
     "inventory",
     "vendor",
     "marketplace_import_job",