feat: add issue labels (DB schema, API, and service)

New labels and issue_labels tables with cascade deletes, unique
per-company name constraint. CRUD routes for labels, label filtering
on issue list, and label sync on issue create/update. All issue
responses now include labels array.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/lib/queryKeys.ts

@@ -14,6 +14,7 @@ export const queryKeys = {
   },
   issues: {
     list: (companyId: string) => ["issues", companyId] as const,
+    labels: (companyId: string) => ["issues", companyId, "labels"] as const,
     listByProject: (companyId: string, projectId: string) =>
       ["issues", companyId, "project", projectId] as const,
     detail: (id: string) => ["issues", "detail", id] as const,