C5 · location_reddit_user pivot denormalization (PR5)

BUILD COMPLETE — C5/PR5 shipped on `feature/authors-optimization` (isolated worktree, cloned DB `lounge_feature_authors_optimization`). Single commit **1414f544** (trailer `Brief: #251`). No query change — durable refactor only, per doc 10 §2.6. **Migration** `2026_07_05_000000_add_denorm_columns_to_location_reddit_user.php` — adds `total_karma INT NULL`, `last_seen_at TIMESTAMP NULL`, `registered_at TIMESTAMP NULL`, `is_visible TINYINT UNSIGNED NOT NULL DEFAULT 0`, `has_verified_flair TINYINT UNSIGNED NOT NULL DEFAULT 0` via raw `ALTER … ALGORITHM=INSTANT`. **Verified INSTANT** on MySQL 8.4.9: ran in 56ms over 197K rows; `information_schema.INNODB_TABLES.TOTAL_ROW_VERSIONS` = 1 (row-version bump, no rebuild). NO composite indexes (that's C6). **Single fold rule** `RedditUser::foldAuthorsVisibility(bool $indexed,$spamBanned,$disabled,$suspended)` — mirrors `RedditUserIndex::render()` default filter (`indexed=1 AND spam_banned_at/disabled_at/suspended_at all NULL`; deliberately NOT shouldBeIndexed(), which uses purged_at). Backing the pivot's `is_visible`. **Sync surface (every write path covered):** - `RedditUser::isVisibleForAuthorsIndex()` + `syncLocationPivotDenormColumns()` (one indexed ~1.85-row UPDATE), called from a new `wasChanged([...])` block in the existing `static::updated` hook. - `LastSeenUpdateJob` — the mass builder `->update()` bypasses model events, so it now JOIN-copies the just-written `reddit_users.last_seen_at` straight onto the pivot (`syncPivotLastSeen()`), NULL-safe, converges with reconcile. - `RedditPost::upsertLocationPivot()` — seeds denorm on INSERT via `locationPivotDenormSeed()` (one PK lookup) so new pivot rows are immediately visible/orderable instead of landing at DEFAULTs. - `BackfillLocationRedditUserPivot` / `ReconcileLocationRedditUserPivot` raw INSERTs — seed denorm too. Used **bare functional-dependency columns (u.id PK = rp.reddit_user_id in GROUP BY), NOT MAX()** — MAX() coerced NULL TIMESTAMP → '0000-00-00' through the aggregation temp table and tripped NO_ZERO_DATE strict mode (caught by the existing backfill test; fixed). **Commands:** - `lounge:backfill-location-reddit-user-denorm` — chunked join-UPDATE over the pivot, resumable by `reddit_user_id` watermark, idempotent (`--chunk`/`--start-id`/`--dry-run`/`--sleep`). One-shot bring-up; run BEFORE C6 builds its indexes. - `lounge:reconcile-location-reddit-user-denorm` — daily drift backstop, chunked, NULL-safe `<=>` mismatch filter (touches only drifted rows → affected-count = drift count), logs a `Log::warning` drift alarm. **Wired INLINE** in `routes/console.php` (`Schedule::command(...)->dailyAt('05:15')->withoutOverlapping()->onOneServer()->runInBackground()`) — NOT a queued job (maintenance supervisor timeout = 15s). Also: `LocationRedditUser` model gains casts + ide-helper docblocks for the 5 columns. **Validation (all green):** ALTER=INSTANT ✓ · backfill sample-match = 0 mismatches vs source on isolated DB (182,477 rows populated) ✓ · reconcile dry-run = 0 drift ✓ · live LastSeenUpdateJob + model-hook + visibility-transition sync confirmed ✓ · new `tests/Feature/Locations/LocationRedditUserDenormSyncTest.php` (5 tests / 32 assertions) + existing pivot/index/scraper suites green ✓ · `pint` clean on full diff ✓ · phpstan clean on all C5 files (note: phpstan is pre-broken on this branch — `phpstan-baseline.neon` references deleted `ProcessLegitEmbeddingResults.php`; unrelated to C5, verified via temp config). Backfill must be run on prod before C6 index creation.

Child of /authors optimization epic (#239). Build in `feature/authors-optimization` worktree. Implement **PR5** (doc 10 §2/§6): add 5 denorm cols (`total_karma`, `last_seen_at`, `registered_at`, `is_visible`, `has_verified_flair`; INSTANT) to the 7.9M-row `location_reddit_user` pivot; backfill; model sync hook (`RedditUser::updated` + `syncLocationPivotDenormColumns`). **⚠ corrections (doc 10 §2.6):** seed denorm from `LastSeenUpdateJob` (mass-update bypasses model events) AND from the raw-INSERT maintenance commands (`BackfillLocationRedditUserPivot` / `ReconcileLocationRedditUserPivot`); daily reconcile via **INLINE scheduler, NOT a queued job** (maintenance queue = 15s timeout). No query change yet — durable refactor. Independent, but the backfill must complete before C6 builds its indexes. Validation: `ALTER` reports INSTANT; post-backfill pivot rows match source for a sample; reconcile ~0 drift; trigger a `LastSeenUpdateJob` and confirm the pivot copy updates.