docs(16): create multi-user data model phase plan

.planning/ROADMAP.md

@@ -145,7 +145,12 @@ Plans:
   3. Existing data from the single-user era is assigned to the original user account after migration
   4. MCP tools return only data belonging to the authenticated API key's owner
   5. Each user has independent settings (weight unit, onboarding state) that do not affect other users
-**Plans**: TBD
+**Plans:** 4 plans
+Plans:
+- [ ] 16-01-PLAN.md — Schema (pgTable + users table + userId columns), auth middleware userId resolution, test helper
+- [ ] 16-02-PLAN.md — All services updated with userId parameter and per-user query scoping
+- [ ] 16-03-PLAN.md — Routes + MCP tools wired to pass userId from context to services
+- [ ] 16-04-PLAN.md — All tests updated with userId, cross-user isolation tests added
 
 ### Phase 17: Object Storage
 **Goal**: Images are stored in and served from MinIO instead of the local filesystem
@@ -190,6 +195,6 @@ Plans:
 | 13. Setup Impact Preview | v1.3 | 0/2 | Not started | - |
 | 14. PostgreSQL Migration | v2.0 | 0/? | Not started | - |
 | 15. External Authentication | v2.0 | 1/1 | Complete    | 2026-04-04 |
-| 16. Multi-User Data Model | v2.0 | 0/? | Not started | - |
+| 16. Multi-User Data Model | v2.0 | 0/4 | Not started | - |
 | 17. Object Storage | v2.0 | 0/? | Not started | - |
 | 18. Global Items & Public Profiles | v2.0 | 0/? | Not started | - |