feat(go): IO layer behind interfaces (M4)

- io/attendance: CSV-over-public-URL client + Fake for adult/junior tabs
- io/drive: Drive v3 modifiedTime client + Fake
- io/sheets: Sheets v4 client (GetValues/AppendValues/BatchUpdateValues/
  WriteHeader/SortByDateColumn) + Fake with call-capture
- io/cache: Drive-modifiedTime-gated FileCache; two TTL knobs; atomic
  writes; generic Get[T]; Python-compatible JSON format; Flush()
- io/fio: Client interface backed by Fio REST API (apiClient) and HTML
  scraper (transparentClient); Fake; testdata fixtures
- membership/sources: NewSources wires attendance CSV + Sheets + cache
  into LoadAdults/LoadJuniors/LoadTransactions/LoadExceptions; Czech
  month parsing + merged-month maps
- banksync: SyncToSheets (SHA-256 dedup, optional sort) and
  InferPayments ([?] review prefix, dry-run) — tested with fakes
- cmd/fuj: sync and infer subcommands wired; fees and reconcile use
  real NewSources; go.mod gains google.golang.org/api + x/net
- gofumpt extra-rules applied across all packages; lint clean

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

go/go.mod

@@ -2,4 +2,33 @@ module fuj-management/go
 
 go 1.26.1
 
-require golang.org/x/text v0.36.0
+require (
+	golang.org/x/net v0.53.0
+	golang.org/x/text v0.36.0
+	google.golang.org/api v0.278.0
+)
+
+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/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/felixge/httpsnoop v1.0.4 // indirect
+	github.com/go-logr/logr v1.4.3 // indirect
+	github.com/go-logr/stdr v1.2.2 // indirect
+	github.com/google/s2a-go v0.1.9 // indirect
+	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
+	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
+	golang.org/x/crypto v0.50.0 // indirect
+	golang.org/x/oauth2 v0.36.0 // indirect
+	golang.org/x/sys v0.43.0 // indirect
+	google.golang.org/genproto/googleapis/rpc v0.0.0-20260427160629-7cedc36a6bc4 // indirect
+	google.golang.org/grpc v1.80.0 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
+)