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>

migrations/012_nullable_label_milestone.sql

@@ -0,0 +1,65 @@
+-- Migration 012: Make label_name and milestone_title nullable
+-- GitLab returns null for these when the referenced label/milestone has been deleted.
+
+-- Recreate resource_label_events with nullable label_name
+CREATE TABLE resource_label_events_new (
+    id INTEGER PRIMARY KEY,
+    gitlab_id INTEGER NOT NULL,
+    project_id INTEGER NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    issue_id INTEGER REFERENCES issues(id) ON DELETE CASCADE,
+    merge_request_id INTEGER REFERENCES merge_requests(id) ON DELETE CASCADE,
+    action TEXT NOT NULL CHECK (action IN ('add', 'remove')),
+    label_name TEXT,
+    actor_gitlab_id INTEGER,
+    actor_username TEXT,
+    created_at INTEGER NOT NULL,
+    CHECK (
+        (issue_id IS NOT NULL AND merge_request_id IS NULL) OR
+        (issue_id IS NULL AND merge_request_id IS NOT NULL)
+    )
+);
+
+INSERT INTO resource_label_events_new
+SELECT * FROM resource_label_events;
+
+DROP TABLE resource_label_events;
+ALTER TABLE resource_label_events_new RENAME TO resource_label_events;
+
+CREATE UNIQUE INDEX uq_label_events_gitlab ON resource_label_events(gitlab_id, project_id);
+CREATE INDEX idx_label_events_issue ON resource_label_events(issue_id) WHERE issue_id IS NOT NULL;
+CREATE INDEX idx_label_events_mr ON resource_label_events(merge_request_id) WHERE merge_request_id IS NOT NULL;
+CREATE INDEX idx_label_events_created ON resource_label_events(created_at);
+
+-- Recreate resource_milestone_events with nullable milestone_title
+CREATE TABLE resource_milestone_events_new (
+    id INTEGER PRIMARY KEY,
+    gitlab_id INTEGER NOT NULL,
+    project_id INTEGER NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+    issue_id INTEGER REFERENCES issues(id) ON DELETE CASCADE,
+    merge_request_id INTEGER REFERENCES merge_requests(id) ON DELETE CASCADE,
+    action TEXT NOT NULL CHECK (action IN ('add', 'remove')),
+    milestone_title TEXT,
+    milestone_id INTEGER,
+    actor_gitlab_id INTEGER,
+    actor_username TEXT,
+    created_at INTEGER NOT NULL,
+    CHECK (
+        (issue_id IS NOT NULL AND merge_request_id IS NULL) OR
+        (issue_id IS NULL AND merge_request_id IS NOT NULL)
+    )
+);
+
+INSERT INTO resource_milestone_events_new
+SELECT * FROM resource_milestone_events;
+
+DROP TABLE resource_milestone_events;
+ALTER TABLE resource_milestone_events_new RENAME TO resource_milestone_events;
+
+CREATE UNIQUE INDEX uq_milestone_events_gitlab ON resource_milestone_events(gitlab_id, project_id);
+CREATE INDEX idx_milestone_events_issue ON resource_milestone_events(issue_id) WHERE issue_id IS NOT NULL;
+CREATE INDEX idx_milestone_events_mr ON resource_milestone_events(merge_request_id) WHERE merge_request_id IS NOT NULL;
+CREATE INDEX idx_milestone_events_created ON resource_milestone_events(created_at);
+
+-- Update schema version
+INSERT INTO schema_version (version, applied_at, description)
+VALUES (12, strftime('%s', 'now') * 1000, 'Make label_name and milestone_title nullable for deleted labels/milestones');