[2.x] fix: Batch policy state loading to avoid one pivot query per serialized user

[imorland]

Problem

On endpoints that serialize many users (most visibly a discussion list viewed by a privileged actor), fof/terms issued one fof_terms_policy_user pivot query per distinct user on the page (~30 queries for a 20-discussion list on a busy forum). Two separate paths caused this:

1. The per-user policy state fields (fofTermsPoliciesState, fofTermsPoliciesHasUpdate, fofTermsPoliciesMustAccept) each lazy-loaded the user's accepted policies.
2. The permission group processor: any hasPermission()/can() call on a serialized user (e.g. another extension's per-user serializer flag) computes that user's permission groups, and the processor then checks that user's acceptance state to apply the guest downgrade — one lazy pivot query per user.

Fix

1. The state fields now defer their values (Flarum's deferred field resolution) and buffer users via a new PolicyStateBuffer; the first resolved value loads the accepted policies of all buffered users in a single query — mirroring core's EloquentBuffer approach.
2. The discussion Index/Show, posts Index, and users Index endpoints now eager load fofTermsPolicies alongside the users they already load, so the permission group processor finds the relation in place. (posts.user.* is deliberately not eager loaded on discussion Show: endpoint eager loads apply to primary models via loadMissing(), which would load a discussion's entire posts relation. Post stream users are covered by the posts Index endpoint.)

Measured

Real forum, admin actor, /api/discussions?page[limit]=20: extension queries 32 → 3 (constant, regardless of page size), total request queries 149 → 120.

Tests

New PolicyStateQueryCountTest pins the constants: 2 pivot queries for a privileged actor over 5 distinct users, +1 for the actor's own enforcement check, and a test registering an ad-hoc per-user hasPermission() field to simulate the second path.

Related: the core half of this investigation was flarum/framework#4839.

Fixes #49.