docs(16-01): complete multi-user data model foundation plan

- Add 16-01-SUMMARY.md with schema, middleware, and test changes
- Update STATE.md with phase 16 progress and decisions
- Update ROADMAP.md with plan progress (1/4 complete)
- Mark MULTI-01, MULTI-04, MULTI-06 complete in REQUIREMENTS.md

.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:
+- [x] 16-01-PLAN.md — Schema foundation: users table, userId columns, auth middleware, test helper
+- [ ] 16-02-PLAN.md — Service layer userId scoping
+- [ ] 16-03-PLAN.md — Route handlers userId extraction
+- [ ] 16-04-PLAN.md — Test suite updates
 
 ### 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 | 0/? | Not started | - |
-| 16. Multi-User Data Model | v2.0 | 0/? | Not started | - |
+| 16. Multi-User Data Model | v2.0 | 1/4 | In progress | - |
 | 17. Object Storage | v2.0 | 0/? | Not started | - |
 | 18. Global Items & Public Profiles | v2.0 | 0/? | Not started | - |