feat: implement achievements

cmd/migrate/migrations/000002_create_achievements_tables.up.sql

@@ -0,0 +1,20 @@
+CREATE TABLE IF NOT EXISTS achievements (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    name VARCHAR(255) NOT NULL,
+    description TEXT NOT NULL,
+    icon_url VARCHAR(512),
+    created_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS user_achievements (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    achievement_id UUID NOT NULL REFERENCES achievements(id) ON DELETE CASCADE,
+    received_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    UNIQUE(user_id, achievement_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_achievements_user_id ON user_achievements(user_id);
+CREATE INDEX IF NOT EXISTS idx_user_achievements_achievement_id ON user_achievements(achievement_id);
+