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/labels.ts

@@ -0,0 +1,18 @@
+import { pgTable, uuid, text, timestamp, index, uniqueIndex } from "drizzle-orm/pg-core";
+import { companies } from "./companies.js";
+
+export const labels = pgTable(
+  "labels",
+  {
+    id: uuid("id").primaryKey().defaultRandom(),
+    companyId: uuid("company_id").notNull().references(() => companies.id, { onDelete: "cascade" }),
+    name: text("name").notNull(),
+    color: text("color").notNull(),
+    createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
+    updatedAt: timestamp("updated_at", { withTimezone: true }).notNull().defaultNow(),
+  },
+  (table) => ({
+    companyIdx: index("labels_company_idx").on(table.companyId),
+    companyNameIdx: uniqueIndex("labels_company_name_idx").on(table.companyId, table.name),
+  }),
+);