fix: sync audit log model with database schema

ROOT CAUSE: Model was out of sync with actual database schema
- Database: created_at is TIMESTAMP WITH TIME ZONE
- Model: Was incorrectly defined as String(50)
- Schema: Expected str but received datetime objects

FIXES:
1. Changed model created_at from String to DateTime(timezone=True)
2. Changed schema created_at from str to datetime
3. Added json_encoders to properly serialize datetime to ISO format

This fixes all 87 validation errors in audit logs endpoint.

src/app/models/audit_log.py

@@ -45,7 +45,8 @@ class AuditLog(Base):
     additional_metadata = Column(JSONB, default=dict)
     
     # Timestamp
-    created_at = Column(String(50), default=lambda: datetime.utcnow().isoformat())
+    from sqlalchemy import DateTime
+    created_at = Column(DateTime(timezone=True), default=datetime.utcnow)
     
     def __repr__(self):
         return f"<AuditLog(action='{self.action}', user='{self.user_email}', entity='{self.entity_type}')>"

src/app/schemas/audit_log.py

@@ -34,10 +34,13 @@ class AuditLogResponse(BaseModel):
     latitude: Optional[str] = None
     longitude: Optional[str] = None
     additional_metadata: Optional[Dict[str, Any]] = None
-    created_at: str
+    created_at: datetime
     
     class Config:
         from_attributes = True
+        json_encoders = {
+            datetime: lambda v: v.isoformat() if v else None
+        }
 
 
 class AuditLogListResponse(BaseModel):