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/database/__init__.py

@@ -26,6 +26,14 @@ from .letzshop import (
     VendorLetzshopCredentials,
 )
 from .marketplace_import_job import MarketplaceImportError, MarketplaceImportJob
+from .message import (
+    Conversation,
+    ConversationParticipant,
+    ConversationType,
+    Message,
+    MessageAttachment,
+    ParticipantType,
+)
 from .marketplace_product import (
     DigitalDeliveryMethod,
     MarketplaceProduct,
@@ -96,4 +104,11 @@ __all__ = [
     "LetzshopFulfillmentQueue",
     "LetzshopSyncLog",
     "LetzshopHistoricalImportJob",
+    # Messaging
+    "Conversation",
+    "ConversationParticipant",
+    "ConversationType",
+    "Message",
+    "MessageAttachment",
+    "ParticipantType",
 ]