feat(db): create labels table for global slide labels

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

database/migrations/2026_05_03_100100_create_labels_table.php

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+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class () extends Migration {
+    public function up(): void
+    {
+        Schema::create('labels', function (Blueprint $table) {
+            $table->id();
+            $table->string('name')->unique();
+            $table->string('color', 7)->nullable();
+            $table->timestamp('hidden_at')->nullable();
+            $table->timestamp('last_imported_at')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('labels');
+    }
+};

tests/Feature/Migrations/LabelsTableTest.php

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+<?php
+
+use Illuminate\Support\Facades\DB;
+use Illuminate\Support\Facades\Schema;
+
+test('labels table has expected columns', function () {
+    expect(Schema::hasTable('labels'))->toBeTrue();
+    expect(Schema::hasColumns('labels', ['id', 'name', 'color', 'hidden_at', 'last_imported_at', 'created_at', 'updated_at']))->toBeTrue();
+});
+
+test('labels table enforces unique name', function () {
+    DB::table('labels')->insert(['name' => 'Vers 1', 'color' => '#FF0080', 'created_at' => now(), 'updated_at' => now()]);
+
+    expect(fn () => DB::table('labels')->insert(['name' => 'Vers 1', 'color' => '#000000', 'created_at' => now(), 'updated_at' => now()]))
+        ->toThrow(\Exception::class);
+});
+
+test('labels table allows nullable color', function () {
+    DB::table('labels')->insert(['name' => 'TestLabel', 'color' => null, 'created_at' => now(), 'updated_at' => now()]);
+
+    expect(DB::table('labels')->where('name', 'TestLabel')->value('color'))->toBeNull();
+});