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>

app/exceptions/__init__.py

@@ -101,6 +101,15 @@ from .inventory import (
     NegativeInventoryException,
 )
 
+# Message exceptions
+from .message import (
+    ConversationClosedException,
+    ConversationNotFoundException,
+    MessageAttachmentException,
+    MessageNotFoundException,
+    UnauthorizedConversationAccessException,
+)
+
 # Marketplace import job exceptions
 from .marketplace_import_job import (
     ImportJobAlreadyProcessingException,
@@ -374,4 +383,10 @@ __all__ = [
     "ScanParseException",
     "ViolationOperationException",
     "InvalidViolationStatusException",
+    # Message exceptions
+    "ConversationNotFoundException",
+    "MessageNotFoundException",
+    "ConversationClosedException",
+    "MessageAttachmentException",
+    "UnauthorizedConversationAccessException",
 ]