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/tests/fixtures/api-schema/version.schema.json

@@ -0,0 +1,26 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$ref": "#/$defs/VersionResponse",
+  "$defs": {
+    "VersionResponse": {
+      "properties": {
+        "tag": {
+          "type": "string"
+        },
+        "commit": {
+          "type": "string"
+        },
+        "build_date": {
+          "type": "string"
+        }
+      },
+      "additionalProperties": false,
+      "type": "object",
+      "required": [
+        "tag",
+        "commit",
+        "build_date"
+      ]
+    }
+  }
+}