Clear the permission cache after adding Recent Topics permissions

[avandenberghe]

Closes #194.

Problem
rt_perms::add_permissions() adds the u_rt_* options, the ROLE_USER_FULL role data and the group grants with raw INSERTs. Unlike the migrator's permission tools, it never calls acl_clear_prefetch(). phpBB only rebuilds a user's cached user_permissions when that column is empty, so existing members kept a cached set without u_rt_view. After the extension was enabled, they saw no Recent Topics until something else happened to clear the cache.

Fix
At the end of add_permissions(), clear the cache the way core's v33x\remove_orphaned_roles migration does:

$auth = new \phpbb\auth\auth();
$auth->acl_clear_prefetch();

A migration has no injected $auth, and acl_clear_prefetch() works through globals.

Verification
add_permissions() builds its own auth, so a mock-based unit test can't observe the clear. Instead I ran the real migration step against the local board's database with a boot script: give a member a cached permission set, run add_permissions() from a given file, then measure user_permissions.

Code	Member's cached permissions before → after
current develop33	866 bytes → 866 bytes (stale, the #194 bug)
this PR	866 bytes → 0 bytes (cleared)

After the clear, phpBB rebuilds the member's permissions with u_rt_view = 1.

The unit suite still passes (44 tests, 0 failures); the functional tests run in CI.

Note on existing installs: this fixes fresh enables. Boards that already enabled 3.0.11 have their cache cleared by any later cache purge or permission change, so they don't need a separate migration.

No changelog entry or version bump, per this repo's convention.

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