docs(15-01): complete Logto Docker infrastructure plan

- Create 15-01-SUMMARY.md with execution results
- Update STATE.md with phase 15 position and decisions
- Update ROADMAP.md with plan progress
- Mark AUTH-04 requirement complete

.planning/ROADMAP.md

@@ -189,7 +189,7 @@ Plans:
 | 12. Comparison View | v1.3 | 1/1 | Complete | 2026-03-17 |
 | 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 | - |
+| 15. External Authentication | v2.0 | 1/3 | In Progress|  |
 | 16. Multi-User Data Model | v2.0 | 0/? | Not started | - |
 | 17. Object Storage | v2.0 | 0/? | Not started | - |
 | 18. Global Items & Public Profiles | v2.0 | 0/? | Not started | - |