chore: CHANGELOG and progress tracker for M5.2

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

docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md

@@ -98,7 +98,7 @@ Goal: every external IO (Sheets, Drive, Fio, file cache) accessed through a narr
 Goal: byte-equal JSON between Python and Go for every route. This is the parity contract.
 
 - [x] **M5.1** Hand-author Go structs for `/api/adults`, `/api/juniors`, `/api/payments`, `/api/version` with explicit `json:` tags matching Python keys; emit JSON Schemas via `github.com/invopop/jsonschema` to `tests/fixtures/api-schema/` — `f253e3f`
-- [ ] **M5.2** Implement Go handlers for `/api/*` routes composing `services/*` results into the JSON structs
+- [x] **M5.2** Implement Go handlers for `/api/*` routes composing `services/*` results into the JSON structs — `7d48e8f`
 - [ ] **M5.3** Add Python `/api/X` shadow endpoints in [app.py](app.py): `jsonify(view_model_dict)` — no transformation
 - [ ] **M5.4** Build `cmd/parity/main.go`: hits both backends' `/api/X`, normalizes allowlist (`render_time.total`, `build_meta`), prints `cmp.Diff`. Add `make parity` target