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/schemagen_test.go

@@ -0,0 +1,81 @@
+package api
+
+// schemagen_test.go generates and golden-compares JSON Schema files for every
+// /api/X response type.
+//
+// Normal run (CI): go test ./internal/web/api/... — asserts schemas match committed files.
+// Regenerate:      go test -run TestGenerateSchemas -update ./internal/web/api/...
+
+import (
+	"encoding/json"
+	"flag"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/invopop/jsonschema"
+)
+
+var updateFlag = flag.Bool("update", false, "overwrite api-schema fixture files with freshly generated schemas")
+
+// JSONSchema makes Expected self-describing for the reflector at test time.
+// The method is in a test file and is not compiled into production binaries.
+// It emits oneOf [integer, "?"] to match the custom MarshalJSON behaviour.
+func (Expected) JSONSchema() *jsonschema.Schema {
+	return &jsonschema.Schema{
+		OneOf: []*jsonschema.Schema{
+			{Type: "integer"},
+			{Enum: []any{"?"}},
+		},
+	}
+}
+
+func TestGenerateSchemas(t *testing.T) {
+	r := &jsonschema.Reflector{
+		AllowAdditionalProperties: false,
+	}
+
+	cases := []struct {
+		name string
+		val  any
+	}{
+		{"adults", &AdultsResponse{}},
+		{"juniors", &JuniorsResponse{}},
+		{"payments", &PaymentsResponse{}},
+		{"version", &VersionResponse{}},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			schema := r.Reflect(tc.val)
+
+			got, err := json.MarshalIndent(schema, "", "  ")
+			if err != nil {
+				t.Fatalf("marshal schema: %v", err)
+			}
+			got = append(got, '\n')
+
+			// Path: go/internal/web/api/ → ../../.. → go/ → tests/fixtures/api-schema/
+			path := filepath.Join("..", "..", "..", "tests", "fixtures", "api-schema", tc.name+".schema.json")
+
+			if *updateFlag {
+				if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+					t.Fatalf("mkdir: %v", err)
+				}
+				if err := os.WriteFile(path, got, 0o644); err != nil {
+					t.Fatalf("write schema: %v", err)
+				}
+				t.Logf("wrote %s", path)
+				return
+			}
+
+			want, err := os.ReadFile(path)
+			if err != nil {
+				t.Fatalf("read fixture %s: %v (re-run with -update to generate)", path, err)
+			}
+			if string(got) != string(want) {
+				t.Errorf("schema mismatch for %s; re-run with -update to regenerate", tc.name)
+			}
+		})
+	}
+}