fix(events): Handle nullable label and milestone in resource events

GitLab returns null for the label/milestone fields on resource_label_events
and resource_milestone_events when the referenced label or milestone has
been deleted. This caused deserialization failures during sync.

- Add migration 012 to recreate both event tables with nullable
  label_name, milestone_title, and milestone_id columns (SQLite
  requires table recreation to alter NOT NULL constraints)
- Change GitLabLabelEvent.label and GitLabMilestoneEvent.milestone
  to Option<> in the Rust types
- Update upsert functions to pass through None values correctly
- Add tests for null label and null milestone deserialization

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

tests/gitlab_types_tests.rs

@@ -729,10 +729,11 @@ fn deserializes_label_event_add() {
 
     assert_eq!(event.id, 2001);
     assert_eq!(event.action, "add");
-    assert_eq!(event.label.id, 100);
-    assert_eq!(event.label.name, "bug");
-    assert_eq!(event.label.color, Some("#FF0000".to_string()));
-    assert_eq!(event.label.description, Some("Bug label".to_string()));
+    let label = event.label.expect("label should be Some");
+    assert_eq!(label.id, 100);
+    assert_eq!(label.name, "bug");
+    assert_eq!(label.color, Some("#FF0000".to_string()));
+    assert_eq!(label.description, Some("Bug label".to_string()));
 }
 
 #[test]
@@ -760,8 +761,57 @@ fn deserializes_label_event_remove_null_color() {
         serde_json::from_str(json).expect("Failed to deserialize label remove event");
 
     assert_eq!(event.action, "remove");
-    assert!(event.label.color.is_none());
-    assert!(event.label.description.is_none());
+    let label = event.label.expect("label should be Some");
+    assert!(label.color.is_none());
+    assert!(label.description.is_none());
+}
+
+#[test]
+fn deserializes_label_event_null_label() {
+    let json = r#"{
+        "id": 2003,
+        "user": {
+            "id": 42,
+            "username": "developer",
+            "name": "Dev User"
+        },
+        "created_at": "2024-03-15T10:30:00.000Z",
+        "resource_type": "Issue",
+        "resource_id": 999,
+        "label": null,
+        "action": "remove"
+    }"#;
+
+    let event: GitLabLabelEvent =
+        serde_json::from_str(json).expect("Failed to deserialize label event with null label");
+
+    assert_eq!(event.id, 2003);
+    assert_eq!(event.action, "remove");
+    assert!(event.label.is_none());
+}
+
+#[test]
+fn deserializes_milestone_event_null_milestone() {
+    let json = r#"{
+        "id": 3002,
+        "user": {
+            "id": 42,
+            "username": "developer",
+            "name": "Dev User"
+        },
+        "created_at": "2024-03-15T10:30:00.000Z",
+        "resource_type": "Issue",
+        "resource_id": 999,
+        "milestone": null,
+        "action": "remove"
+    }"#;
+
+    let event: GitLabMilestoneEvent = serde_json::from_str(json)
+        .expect("Failed to deserialize milestone event with null milestone");
+
+    assert_eq!(event.id, 3002);
+    assert_eq!(event.action, "remove");
+    assert!(event.milestone.is_none());
 }
 
 #[test]
@@ -789,9 +839,10 @@ fn deserializes_milestone_event() {
 
     assert_eq!(event.id, 3001);
     assert_eq!(event.action, "add");
-    assert_eq!(event.milestone.id, 200);
-    assert_eq!(event.milestone.iid, 5);
-    assert_eq!(event.milestone.title, "v1.0");
+    let milestone = event.milestone.expect("milestone should be Some");
+    assert_eq!(milestone.id, 200);
+    assert_eq!(milestone.iid, 5);
+    assert_eq!(milestone.title, "v1.0");
 }
 
 #[test]