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/api/issues.ts

@@ -1,4 +1,4 @@
-import type { Approval, Issue, IssueAttachment, IssueComment } from "@paperclip/shared";
+import type { Approval, Issue, IssueAttachment, IssueComment, IssueLabel } from "@paperclip/shared";
 import { api } from "./client";
 
 export const issuesApi = {
@@ -8,6 +8,10 @@ export const issuesApi = {
     const qs = params.toString();
     return api.get<Issue[]>(`/companies/${companyId}/issues${qs ? `?${qs}` : ""}`);
   },
+  listLabels: (companyId: string) => api.get<IssueLabel[]>(`/companies/${companyId}/labels`),
+  createLabel: (companyId: string, data: { name: string; color: string }) =>
+    api.post<IssueLabel>(`/companies/${companyId}/labels`, data),
+  deleteLabel: (id: string) => api.delete<IssueLabel>(`/labels/${id}`),
   get: (id: string) => api.get<Issue>(`/issues/${id}`),
   create: (companyId: string, data: Record<string, unknown>) =>
     api.post<Issue>(`/companies/${companyId}/issues`, data),

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,