refactor: migrate modules from re-exports to canonical implementations

Move actual code implementations into module directories:
- orders: 5 services, 4 models, order/invoice schemas
- inventory: 3 services, 2 models, 30+ schemas
- customers: 3 services, 2 models, customer schemas
- messaging: 3 services, 2 models, message/notification schemas
- monitoring: background_tasks_service
- marketplace: 5+ services including letzshop submodule
- dev_tools: code_quality_service, test_runner_service
- billing: billing_service
- contracts: definition.py

Legacy files in app/services/, models/database/, models/schema/
now re-export from canonical module locations for backwards
compatibility. Architecture validator passes with 0 errors.

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

app/modules/messaging/models/admin_notification.py

@@ -0,0 +1,54 @@
+# app/modules/messaging/models/admin_notification.py
+"""
+Admin notification database model.
+
+This model handles admin-specific notifications for system alerts and warnings.
+"""
+
+from sqlalchemy import (
+    Boolean,
+    Column,
+    DateTime,
+    ForeignKey,
+    Integer,
+    JSON,
+    String,
+    Text,
+)
+from sqlalchemy.orm import relationship
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class AdminNotification(Base, TimestampMixin):
+    """
+    Admin-specific notifications for system alerts and warnings.
+
+    Different from vendor/customer notifications - these are for platform
+    administrators to track system health and issues requiring attention.
+    """
+
+    __tablename__ = "admin_notifications"
+
+    id = Column(Integer, primary_key=True, index=True)
+    type = Column(
+        String(50), nullable=False, index=True
+    )  # system_alert, vendor_issue, import_failure
+    priority = Column(
+        String(20), default="normal", index=True
+    )  # low, normal, high, critical
+    title = Column(String(200), nullable=False)
+    message = Column(Text, nullable=False)
+    is_read = Column(Boolean, default=False, index=True)
+    read_at = Column(DateTime, nullable=True)
+    read_by_user_id = Column(Integer, ForeignKey("users.id"), nullable=True)
+    action_required = Column(Boolean, default=False, index=True)
+    action_url = Column(String(500))  # Link to relevant admin page
+    notification_metadata = Column(JSON)  # Additional contextual data
+
+    # Relationships
+    read_by = relationship("User", foreign_keys=[read_by_user_id])
+
+    def __repr__(self):
+        return f"<AdminNotification(id={self.id}, type='{self.type}', priority='{self.priority}')>"