feat(user): configurable foreign key type (bigint/uuid/ulid)

[cjmellor]

Summary

Adds level-up.user.foreign_key_type so host apps whose User model uses HasUuids or HasUlids can install the package without hand-editing every user-FK column.

• New config key level-up.user.foreign_key_type (default 'bigint', accepts 'uuid' or 'ulid')
• New helper LevelUp\Experience\Support\UserForeignKey::on($table) that returns the right column type and is callable from migrations
• All six user-FK migration stubs now route through the helper: experiences, streaks, achievement_user, challenge_user, experience_audits, streak_histories
• UUID + ULID fixtures and end-to-end flow tests covering experience, streak and challenge flows

Migration

This is opt-in and backwards-compatible. The default 'bigint' keeps the current foreignId(...) behaviour, so existing installs see no change at all. Only set foreign_key_type to 'uuid'/'ulid' on a fresh install where the host User PK matches.

// config/level-up.php
'user' => [
    'foreign_key' => 'user_id',
    'foreign_key_type' => 'uuid', // 'bigint' (default) | 'uuid' | 'ulid'
    'model' => App\Models\User::class,
    'users_table' => 'users',
],

Out of scope (known limitations)

• Package-internal PKs (e.g. experiences.id, levels.id) stay bigint. The pain point this PR addresses is the user-FK boundary; internal PKs only matter to package code which already treats keys polymorphically through Eloquent.
• multiplier_scopes.scopeable_id is still unsignedBigInteger, so scoping a Multiplier directly to a UUID/ULID user via morphedByMany won't match. Tier scopes are unaffected. Tracked as a follow-up.