feat(appview): theme read API endpoints (ATB-55) (#86)
* docs: add ATB-55 theme read API design doc
Records approved design for themes table, theme_policies table,
theme_policy_available_themes join table, firehose indexer configs,
and GET /api/themes + GET /api/themes/:rkey + GET /api/theme-policy endpoints.
* docs: add ATB-55 theme API implementation plan
* feat(db): add themes, theme_policies, theme_policy_available_themes tables
Generate Postgres (0013) and SQLite (0001) migrations for the three new
theme tables. Build @atbb/db to verify schema compiles correctly.
* feat(appview): add GET /api/themes, /api/themes/:rkey, /api/theme-policy endpoints (ATB-55)
* feat(appview): index space.atbb.forum.theme and themePolicy from firehose (ATB-55)
* docs(bruno): add Themes API collection (ATB-55)