feat(go): M5.1 — hand-author /api/* wire types + JSON Schemas

Add internal/web/api package with Go structs for every /api/X route:
AdultsResponse, JuniorsResponse, PaymentsResponse, VersionResponse.
All fields carry explicit json: tags matching the Python view-model keys.

Key design choices:
- member_data / month_labels / raw_payments are nested objects (not
  the pre-serialised JSON strings used in Jinja templates)
- Expected{Value int; Unknown bool} with custom MarshalJSON emits int
  or the string "?" for junior single-attendance months
- RawTransaction covers the full 11-column payments sheet row

schemagen_test.go reflects all four response types via
github.com/invopop/jsonschema and golden-compares against committed
schemas in tests/fixtures/api-schema/. The JSONSchema() method on
Expected lives in the test file so the prod binary has no jsonschema
dependency.

Closes M5.1 in docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/api/payments.go

@@ -0,0 +1,9 @@
+package api
+
+// PaymentsResponse is the JSON contract for GET /api/payments.
+type PaymentsResponse struct {
+	GroupedPayments map[string][]RawTransaction `json:"grouped_payments"` // person name → rows
+	SortedPeople    []string                    `json:"sorted_people"`
+	AttendanceURL   string                      `json:"attendance_url"`
+	PaymentsURL     string                      `json:"payments_url"`
+}