feat(appview): theme write API endpoints — POST/PUT/DELETE themes + PUT theme-policy (ATB-57) (#87)
* docs: add design doc for ATB-57 theme write API endpoints
* docs: add implementation plan for ATB-57 theme write API endpoints
* feat(appview): add manageThemes permission to Admin role (ATB-57)
* feat(appview): POST /api/admin/themes — create theme on Forum PDS (ATB-57)
* test(appview): add 401/403/PDS-failure tests for POST /api/admin/themes (ATB-57)
* feat(appview): PUT /api/admin/themes/:rkey — update theme on Forum PDS (ATB-57)
* feat(appview): DELETE /api/admin/themes/:rkey — delete theme, 409 if default (ATB-57)
* test(appview): assert 409 error body in dark-theme default check (ATB-57)
* test(appview): verify deleteRecord called with exact theme args (ATB-57)
* feat(appview): PUT /api/admin/theme-policy — upsert policy singleton on Forum PDS (ATB-57)
* test(appview): add theme-policy update path and updatedAt assertions (ATB-57)
* refactor(appview): strengthen theme-policy type guards and test assertions (ATB-57)
* docs(bruno): add Admin Themes collection for ATB-57 write endpoints
* docs: mark ATB-57 plan docs complete, move to docs/plans/complete/
* fix(appview): add 503 ForumAgent-not-authenticated tests; fix Bruno error code docs (ATB-57)