C1 · /authors whereExists location rewrite (PR1)

C1/PR1 BUILD COMPLETE — whereHas → whereExists rewrite (doc 10 §5). **Context:** C6/PR6 (40b2e3ad) already made the default single-location browse pivot-first, so the whereHas('locations') that §5 targets now lives ONLY in the legacy reddit_users-driven fallback branch (admin toggles / only_indexed off / search + location). The code even carried a comment flagging "the whereExists simplification for this branch is doc §5, a separate PR" — that's this brief. **Change (commit e2affdc9, branch feature/authors-optimization):** - `app/Livewire/RedditUserIndex.php` — replaced the fallback `whereHas('locations', fn ($q) => $q->where('locations.id', $id))` with a `whereExists` straight on the `location_reddit_user` pivot (selectRaw('1') → from('location_reddit_user') → whereColumn(reddit_user_id = reddit_users.id) → where(location_id = ?)). Updated the surrounding comment to document the plan-equivalence (pivot PK guarantees ≤1 row per (location,user), no fan-out). Pivot-first fast path untouched. - `tests/Feature/Livewire/RedditUserIndexTest.php` — added `test_legacy_location_fallback_scopes_to_resolved_location`: seeds authors at two distinct locations, forces the legacy path via only_indexed=false, asserts the resolved-location author appears and the other-location author is excluded (locks in the exclusion boundary the semijoin must preserve — existing tests only checked inclusion). **Validation (brief criteria):** - `toSql()` of the rewritten semijoin confirms the `locations` table is GONE from the emitted plan: `... where exists (select 1 from location_reddit_user where location_reddit_user.reddit_user_id = reddit_users.id and location_reddit_user.location_id = ?)`. has_locations_join=NO. - Result rows identical: full `RedditUserIndexTest` suite green — 5 passed / 24 assertions (4 pre-existing + 1 new exclusion test). - `./vendor/bin/pint --test` on both changed files: passed. All work in the isolated worktree (project 57); MAIN + its DB untouched. No migrations (pure query rewrite). Commit SHA: **e2affdc9**.

Child of /authors optimization epic (#239). Build in `feature/authors-optimization` worktree. Implement **PR1** (docs/authors-optimization/10-perf-and-indexing.md §5): drop the redundant `INNER JOIN locations` from the `/authors` location semijoin — rewrite `whereHas('locations', …)` to `whereExists` (plan-equivalent, strict win). Touches `RedditUserIndex.php`. Safe quick win, independent (no deps). Validation: `EXPLAIN FORMAT=TREE` the location query — confirm `locations` is gone from the plan; result rows identical.