fix: resolve plan view crash from duplicate children and tiptap link conflict

Three fixes:

1. GetChildItems returned duplicate rows when an item was linked to a parent
   via multiple link types (e.g. both "parent" and "implements"), causing
   Svelte's {#each} to throw each_key_duplicate. Added SELECT DISTINCT.

2. StarterKit v3.20.4 now includes Link by default, conflicting with our
   custom SafeLink extension. Disabled StarterKit's built-in link.

3. Migration runner now tolerates "duplicate column name" errors on
   ALTER TABLE ADD COLUMN, making migrations idempotent when partially
   applied (e.g. server crash mid-migration).

internal/store/items.go

@@ -1317,7 +1317,7 @@ func (s *Store) GetAllItemProgress(workspaceID, collectionSlug string) ([]ItemPr
 // via item_links. Returns children from any collection.
 func (s *Store) GetChildItems(parentItemID string) ([]models.Item, error) {
 	rows, err := s.db.Query(s.q(fmt.Sprintf(`
-		SELECT i.id, i.workspace_id, i.collection_id, i.title, i.slug, i.content, i.fields, i.tags,
+		SELECT DISTINCT i.id, i.workspace_id, i.collection_id, i.title, i.slug, i.content, i.fields, i.tags,
 		       i.pinned, i.sort_order, i.parent_id, i.assigned_user_id, i.agent_role_id, i.role_sort_order,
 		       i.created_by, i.last_modified_by, i.source,
 		       i.item_number, i.created_at, i.updated_at,

internal/store/store.go

@@ -255,7 +255,15 @@ func execMulti(db *sql.DB, sqlText string) error {
 		stmt := strings.TrimSpace(sqlText[:end+1])
 		if stmt != "" && stmt != ";" {
 			if _, err := db.Exec(stmt); err != nil {
-				return fmt.Errorf("exec migration statement: %w\nStatement: %.200s", err, stmt)
+				// Tolerate "duplicate column name" errors from ALTER TABLE ADD COLUMN.
+				// This makes migrations idempotent when partially applied (e.g. server
+				// crashed after adding a column but before recording the migration).
+				upper := strings.ToUpper(strings.TrimSpace(stmt))
+				isDupCol := strings.Contains(err.Error(), "duplicate column name")
+				isAddCol := strings.HasPrefix(upper, "ALTER TABLE") && strings.Contains(upper, "ADD COLUMN")
+				if !(isDupCol && isAddCol) {
+					return fmt.Errorf("exec migration statement: %w\nStatement: %.200s", err, stmt)
+				}
 			}
 		}
 		sqlText = sqlText[end+1:]

web/src/lib/components/editor/Editor.svelte

@@ -230,6 +230,7 @@
 		const extensions = [
 			StarterKit.configure({
 				codeBlock: false,
+				link: false, // We use our own SafeLink extension below
 			}),
 			MermaidCodeBlock.configure({
 				HTMLAttributes: { class: 'code-block' },