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/go.mod

@@ -3,6 +3,7 @@ module fuj-management/go
 go 1.26.1
 
 require (
+	github.com/invopop/jsonschema v0.14.0
 	golang.org/x/net v0.53.0
 	golang.org/x/text v0.36.0
 	google.golang.org/api v0.278.0
@@ -12,6 +13,8 @@ require (
 	cloud.google.com/go/auth v0.20.0 // indirect
 	cloud.google.com/go/auth/oauth2adapt v0.2.8 // indirect
 	cloud.google.com/go/compute/metadata v0.9.0 // indirect
+	github.com/bahlo/generic-list-go v0.2.0 // indirect
+	github.com/buger/jsonparser v1.1.2 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/felixge/httpsnoop v1.0.4 // indirect
 	github.com/go-logr/logr v1.4.3 // indirect
@@ -20,11 +23,13 @@ require (
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.3.15 // indirect
 	github.com/googleapis/gax-go/v2 v2.22.0 // indirect
+	github.com/pb33f/ordered-map/v2 v2.3.1 // indirect
 	go.opentelemetry.io/auto/sdk v1.2.1 // indirect
 	go.opentelemetry.io/contrib/instrumentation/net/http/otelhttp v0.67.0 // indirect
 	go.opentelemetry.io/otel v1.43.0 // indirect
 	go.opentelemetry.io/otel/metric v1.43.0 // indirect
 	go.opentelemetry.io/otel/trace v1.43.0 // indirect
+	go.yaml.in/yaml/v4 v4.0.0-rc.2 // indirect
 	golang.org/x/crypto v0.50.0 // indirect
 	golang.org/x/oauth2 v0.36.0 // indirect
 	golang.org/x/sys v0.43.0 // indirect