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",
 ]

app/exceptions/message.py

@@ -0,0 +1,63 @@
+# app/exceptions/message.py
+"""
+Messaging specific exceptions.
+"""
+
+from .base import BusinessLogicException, ResourceNotFoundException, ValidationException
+
+
+class ConversationNotFoundException(ResourceNotFoundException):
+    """Raised when a conversation is not found."""
+
+    def __init__(self, conversation_identifier: str):
+        super().__init__(
+            resource_type="Conversation",
+            identifier=conversation_identifier,
+            message=f"Conversation '{conversation_identifier}' not found",
+            error_code="CONVERSATION_NOT_FOUND",
+        )
+
+
+class MessageNotFoundException(ResourceNotFoundException):
+    """Raised when a message is not found."""
+
+    def __init__(self, message_identifier: str):
+        super().__init__(
+            resource_type="Message",
+            identifier=message_identifier,
+            message=f"Message '{message_identifier}' not found",
+            error_code="MESSAGE_NOT_FOUND",
+        )
+
+
+class ConversationClosedException(BusinessLogicException):
+    """Raised when trying to send message to a closed conversation."""
+
+    def __init__(self, conversation_id: int):
+        super().__init__(
+            message=f"Cannot send message to closed conversation {conversation_id}",
+            error_code="CONVERSATION_CLOSED",
+            details={"conversation_id": conversation_id},
+        )
+
+
+class MessageAttachmentException(ValidationException):
+    """Raised when attachment validation fails."""
+
+    def __init__(self, message: str, details: dict | None = None):
+        super().__init__(
+            message=message,
+            error_code="MESSAGE_ATTACHMENT_INVALID",
+            details=details,
+        )
+
+
+class UnauthorizedConversationAccessException(BusinessLogicException):
+    """Raised when user tries to access a conversation they don't have access to."""
+
+    def __init__(self, conversation_id: int):
+        super().__init__(
+            message=f"You do not have access to conversation {conversation_id}",
+            error_code="CONVERSATION_ACCESS_DENIED",
+            details={"conversation_id": conversation_id},
+        )