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/migrations/0018_flat_sleepwalker.sql

@@ -0,0 +1,26 @@
+CREATE TABLE "issue_labels" (
+	"issue_id" uuid NOT NULL,
+	"label_id" uuid NOT NULL,
+	"company_id" uuid NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	CONSTRAINT "issue_labels_pk" PRIMARY KEY("issue_id","label_id")
+);
+--> statement-breakpoint
+CREATE TABLE "labels" (
+	"id" uuid PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+	"company_id" uuid NOT NULL,
+	"name" text NOT NULL,
+	"color" text NOT NULL,
+	"created_at" timestamp with time zone DEFAULT now() NOT NULL,
+	"updated_at" timestamp with time zone DEFAULT now() NOT NULL
+);
+--> statement-breakpoint
+ALTER TABLE "issue_labels" ADD CONSTRAINT "issue_labels_issue_id_issues_id_fk" FOREIGN KEY ("issue_id") REFERENCES "public"."issues"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "issue_labels" ADD CONSTRAINT "issue_labels_label_id_labels_id_fk" FOREIGN KEY ("label_id") REFERENCES "public"."labels"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "issue_labels" ADD CONSTRAINT "issue_labels_company_id_companies_id_fk" FOREIGN KEY ("company_id") REFERENCES "public"."companies"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+ALTER TABLE "labels" ADD CONSTRAINT "labels_company_id_companies_id_fk" FOREIGN KEY ("company_id") REFERENCES "public"."companies"("id") ON DELETE cascade ON UPDATE no action;--> statement-breakpoint
+CREATE INDEX "issue_labels_issue_idx" ON "issue_labels" USING btree ("issue_id");--> statement-breakpoint
+CREATE INDEX "issue_labels_label_idx" ON "issue_labels" USING btree ("label_id");--> statement-breakpoint
+CREATE INDEX "issue_labels_company_idx" ON "issue_labels" USING btree ("company_id");--> statement-breakpoint
+CREATE INDEX "labels_company_idx" ON "labels" USING btree ("company_id");--> statement-breakpoint
+CREATE UNIQUE INDEX "labels_company_name_idx" ON "labels" USING btree ("company_id","name");