feat(06-01): add config history service with TDD tests

- Service queries router_config_changes JOIN router_config_diffs for timeline
- Returns paginated entries with component, summary, timestamp, diff metadata
- ORDER BY created_at DESC with limit/offset pagination
- 4 tests covering formatting, empty results, pagination, and ordering

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.planning/ROADMAP.md

@@ -106,11 +106,11 @@ Plans:
   3. GET `/api/tenants/{tid}/devices/{did}/config/{snapshot_id}/diff` returns unified diff text
   4. All endpoints enforce RBAC: viewer+ can read history, operator+ required for backup trigger
   5. Endpoints return proper 404 for nonexistent snapshots and 403 for unauthorized access
-**Plans**: TBD
+**Plans**: 2 plans
 
 Plans:
-- [ ] 06-01: Config history timeline endpoint
-- [ ] 06-02: Snapshot view and diff retrieval endpoints with RBAC
+- [ ] 06-01-PLAN.md — Config history timeline endpoint with service, router, and tests
+- [ ] 06-02-PLAN.md — Snapshot view and diff retrieval endpoints with Transit decrypt and RBAC
 
 ### Phase 7: Config History UI
 **Goal**: Device detail page displays a Configuration History section showing a timeline of config changes