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/adults.go

@@ -0,0 +1,42 @@
+package api
+
+// AdultsMonthData is the reconciled ledger for one adult member in one month.
+// Keys match Python's result["members"][name]["months"][YYYY-MM].
+type AdultsMonthData struct {
+	Expected         int             `json:"expected"`
+	OriginalExpected int             `json:"original_expected"`
+	AttendanceCount  int             `json:"attendance_count"`
+	Exception        *ExceptionData  `json:"exception"`
+	Paid             float64         `json:"paid"` // float: proportional allocator may produce fractional CZK
+	Transactions     []MemberTxEntry `json:"transactions"`
+}
+
+// AdultsMemberData is the reconciled ledger for one adult member.
+// Keys match Python's result["members"][name].
+type AdultsMemberData struct {
+	Tier              string                     `json:"tier"`
+	Months            map[string]AdultsMonthData `json:"months"` // YYYY-MM → month data
+	OtherTransactions []MemberOtherEntry         `json:"other_transactions"`
+	TotalBalance      int                        `json:"total_balance"`
+}
+
+// AdultsResponse is the JSON contract for GET /api/adults.
+// MemberData, MonthLabels, and RawPayments correspond to the Python view-model
+// fields member_data, month_labels_json, and raw_payments_json respectively,
+// but as nested objects rather than pre-serialised JSON strings.
+type AdultsResponse struct {
+	Months        []string                    `json:"months"`     // display labels
+	RawMonths     []string                    `json:"raw_months"` // "YYYY-MM"
+	Results       []MemberRow                 `json:"results"`
+	Totals        []TotalCell                 `json:"totals"`
+	MemberData    map[string]AdultsMemberData `json:"member_data"`  // name → ledger
+	MonthLabels   map[string]string           `json:"month_labels"` // YYYY-MM → display label
+	RawPayments   map[string][]RawTransaction `json:"raw_payments"` // name → raw sheet rows
+	Credits       []Credit                    `json:"credits"`
+	Debts         []Credit                    `json:"debts"`
+	Unmatched     []RawTransaction            `json:"unmatched"`
+	AttendanceURL string                      `json:"attendance_url"`
+	PaymentsURL   string                      `json:"payments_url"`
+	BankAccount   string                      `json:"bank_account"`
+	CurrentMonth  string                      `json:"current_month"`
+}