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>

.github/workflows/docker-publish.yml

@@ -2,7 +2,7 @@ name: Publish Container
 
 on:
   push:
-    branches: ["main"]
+    branches: ["main", "beta"]
     tags: ["v*"]
   workflow_dispatch:
 
@@ -50,6 +50,7 @@ jobs:
           images: git.necloud.us/nate/siteforge
           tags: |
             type=raw,value=latest,enable={{is_default_branch}}
+            type=raw,value=beta,enable=${{ github.ref == 'refs/heads/beta' }}
             type=sha
             type=ref,event=tag