Two concurrent series lanes, Complete studies, self-paced finished books; v2.9.0

- Study status 'complete' (manual or auto when all lessons released); stays enrollable and self-paced (no sequential lock, Resume button, Finished Studies hub section)
- Studies get lane (A/B) and expected start date; planned studies show "Starts <date>"; hub shows next lesson date instead of hardcoded pace
- Second concurrent series (homepage card, admin panel) with its own scheduled launch; transitions are per lane and flip the ending study to Complete and the incoming study to Active
- Finished books link to their study: "Study This Book" on playlist page, audio-only link from hub (playlist stays public, no account)
- Admin: bulk release scheduler (every N days), same-day collision warning, linked-study selects
- Weekly digest no longer dumps a finished study's backlog on late enrollees
- CI: build and tag the container as :beta on pushes to the beta branch

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/App.css

@@ -1079,7 +1079,9 @@
 
 .series-grid {
   display: flex;
-  justify-content: center;
+  flex-direction: column;
+  align-items: center;
+  gap: 2rem;
 }
 
 .series-card {
@@ -13122,3 +13124,4 @@
 }
 .study-status-badge.active { background: rgba(201, 168, 76, 0.18); color: #c9a84c; border: 1px solid rgba(201, 168, 76, 0.45); }
 .study-status-badge.planned { background: rgba(255, 255, 255, 0.05); color: #9a9486; border: 1px solid rgba(255, 255, 255, 0.15); }
+.study-status-badge.complete { background: rgba(120, 170, 130, 0.16); color: #8fc79b; border: 1px solid rgba(120, 170, 130, 0.45); }