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>

packages/db/src/schema/index.ts

@@ -12,9 +12,12 @@ export { agentRuntimeState } from "./agent_runtime_state.js";
 export { agentTaskSessions } from "./agent_task_sessions.js";
 export { agentWakeupRequests } from "./agent_wakeup_requests.js";
 export { projects } from "./projects.js";
+export { projectWorkspaces } from "./project_workspaces.js";
 export { projectGoals } from "./project_goals.js";
 export { goals } from "./goals.js";
 export { issues } from "./issues.js";
+export { labels } from "./labels.js";
+export { issueLabels } from "./issue_labels.js";
 export { issueApprovals } from "./issue_approvals.js";
 export { issueComments } from "./issue_comments.js";
 export { assets } from "./assets.js";