docs: add backlog item 999.3 — public access auth model

.planning/ROADMAP.md

@@ -292,3 +292,11 @@ Plans:
 
 Plans:
 - [ ] TBD (promote with /gsd:review-backlog when ready)
+
+### Phase 999.3: Public Access Auth Model (BACKLOG)
+**Goal**: Rework auth so the app is accessible without logging in. Currently all routes require authentication, but public-facing pages (discovery/browse, shared setups, public profiles) should be viewable by unauthenticated users. Auth only required for write operations and personal data.
+**Requirements**: TBD
+**Plans**: TBD
+
+Plans:
+- [ ] TBD (promote with /gsd:review-backlog when ready)