feat(usernames): update unique constraint to composite with type and provider

- Remove single unique constraint on username
  - Add composite unique constraint on username, username_type, and provider_type
  - Update Filament username form validation with custom unique rule
  - Add descriptive validation message for duplicate usernames

database/migrations/2025_12_09_180756_update_usernames_unique_constraint_remove_single_add_composite.php

@@ -0,0 +1,37 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('usernames', function (Blueprint $table) {
+            // Drop the existing unique constraint on 'username' column
+            $table->dropUnique('usernames_username_unique');
+
+            // Add a composite unique constraint on username, username_type, and provider_type
+            // This allows the same username with different type combinations
+            $table->unique(['username', 'username_type', 'provider_type'], 'usernames_username_type_provider_unique');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('usernames', function (Blueprint $table) {
+            // Drop the composite unique constraint
+            $table->dropUnique('usernames_username_type_provider_unique');
+
+            // Restore the original single unique constraint on 'username'
+            $table->unique('username', 'usernames_username_unique');
+        });
+    }
+};