brief sprint C1: relocate Post + Line from dashboard → billboard (no behavior change) — TDD

The Post/Line models always read more like billboard tenants than dashboard ones (1st-person personal vs. 2nd-person provenance feed); the upcoming Brief model needs them in the billboard namespace as the canonical surface they FK into. C1 is a pure relocation w. zero new behavior: 789 ITs + 20 sky/Post FTs green against the moved code.

- billboard.models adds Post (owner + shared_with) + Line (text + post FK), schema mirroring the legacy dashboard models 1:1; Post.get_absolute_url now reverses to `billboard:view_post`.
- billboard.forms adds LineForm + ExistingPostLineForm (moved from dashboard.forms; dashboard/forms.py removed).
- billboard.views absorbs new_post / view_post / share_post / my_posts (templates rendered from apps/billboard/post.html + my_posts.html).
- billboard.urls adds the namespaced routes: /billboard/new-post, /billboard/post/<uuid>/, /billboard/post/<uuid>/share-post, /billboard/users/<uuid>/. dashboard.urls drops the corresponding entries.
- _applet-my-posts + _applet-new-post URL refs now use the billboard: namespace; templates/apps/dashboard/{post,my_posts}.html removed.
- api/serializers + api/views + api/tests/integrated/test_views imports flip dashboard.models → billboard.models (PostSerializer / PostDetailAPI / PostLinesAPI / PostsAPI all retain identifiers — the model rename to Brief lands in C2).
- dashboard/tests/integrated/test_{models,views,forms} + dashboard/tests/unit/test_{models,forms} swap imports; test_views URL strings flip /dashboard/post/ → /billboard/post/, /dashboard/new_post → /billboard/new-post, /dashboard/users/ → /billboard/users/, share_post → share-post (path) / billboard:share_post (reverser). Tests stay in dashboard.tests/ for now — relocation TBD.
- functional_tests/my_posts_page.py URL string flips to /billboard/users/.
- Auto-generated migrations: billboard/0001_initial (CreateModel Post + Line), dashboard/0003_remove_post_* (drops legacy Post + Line), drama/0004_alter_gameevent_verb (incidental — choices field caught up).

This commit drops the dashboard Post/Line tables w/o data preservation; user has confirmed staging-side wipe is acceptable. C2 introduces the Brief model + read-tracking + slide-down banner unification. C3 hooks Note-unlock + share-post-invite + magic-link / invalid-link `messages` calls into the new Brief / banner pipeline.

Code architected by Disco DeDisco <discodedisco@outlook.com>
Git commit message Co-Authored-By:
Claude Sonnet 4.6 <noreply@anthropic.com>

src/apps/api/serializers.py

@@ -1,6 +1,6 @@
 from rest_framework import serializers
 
-from apps.dashboard.models import Line, Post
+from apps.billboard.models import Line, Post
 from apps.lyric.models import User