Make Ollama prompt editable in admin AI settings

- Add prompt column to ai_settings (migration)
- AiSetting: DEFAULT_PROMPT constant + getActivePrompt() (falls back to default if empty)
- ProcessNailPolishImage: reads prompt from DB instead of hardcoded
- Admin view: textarea to edit prompt, shows active prompt, reset-to-default link

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

database/migrations/2026_06_13_000005_add_prompt_to_ai_settings.php

@@ -0,0 +1,22 @@
+<?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::table('ai_settings', function (Blueprint $table) {
+            $table->text('prompt')->nullable()->after('ollama_model');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('ai_settings', function (Blueprint $table) {
+            $table->dropColumn('prompt');
+        });
+    }
+};