feat(studio): /studio content table + bulk visibility / bulk delete

Skeleton for the creator studio surface deferred since Phase 6. The
/studio route lands on a filterable table of the signed-in user's
videos — All / Public / Unlisted / Private / Shorts / Processing-or-
Failed — with headline totals (public / unlisted / private counts,
processing + failed, total views) in a stat strip above.

Each row shows thumbnail + title + stripped description preview +
visibility badge + per-video totals. Row-level checkboxes raise a
toolbar when any row is selected; the toolbar supports:
  - Change visibility across the selection (any of the three visibility
    enums).
  - Delete the selection with a confirm prompt.

Server-side the bulk endpoints owner-filter by Auth::id() so a crafted
request body can't flip or delete anyone else's videos. Response is
JSON { updated | deleted : N } so the page can reload / toast cleanly.

Sidebar now carries a persistent "Studio" link under the You section
alongside History / Liked Videos.

Analytics, stepped upload (Details → Visibility), and schedule remain
future work — this ships the content management surface, nothing else.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

resources/views/layouts/partials/sidebar.blade.php

@@ -57,6 +57,10 @@
             <i class="bi bi-hand-thumbs-up-fill"></i>
             <span>Liked Videos</span>
         </a>
+        <a href="{{ route('studio') }}" class="yt-sidebar-link {{ request()->is('studio*') ? 'active' : '' }}">
+            <i class="bi bi-sliders"></i>
+            <span>Studio</span>
+        </a>
     </div>
     @endauth