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>

docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md

@@ -2,9 +2,9 @@
 
 Companion to [2026-05-03-2349-go-backend-rewrite.md](2026-05-03-2349-go-backend-rewrite.md).
 
-**Current milestone:** M3 — Fixture capture + characterization framework ✅
+**Current milestone:** M4 — IO layer behind interfaces ✅
 **Started:** 2026-05-04
-**Last updated:** 2026-05-06
+**Last updated:** 2026-05-07
 
 ## How to use
 
@@ -80,16 +80,16 @@ Goal: deterministic, PII-free fixture corpus that drives parity tests. Runs in p
 
 Goal: every external IO (Sheets, Drive, Fio, file cache) accessed through a narrow Go interface with both a real and a fake implementation.
 
-- [ ] **M4.1** Design IO interfaces (`SheetsClient`, `DriveClient`, `FioClient`, `FileCache`) + in-memory fakes seeded from M3 fixtures
-- [ ] **M4.2** `internal/io/sheets` — Google client (read + append + batchUpdate); integration test against a separate test sheet (NOT prod)
-- [ ] **M4.3** `internal/io/drive` — Drive `modifiedTime` client + integration test
-- [ ] **M4.4** `internal/io/fio` — API JSON impl (token-based); parses by hardcoded `column0..column22` indices matching [fio_utils.py](scripts/fio_utils.py)
-- [ ] **M4.5** `internal/io/fio` — transparent-page HTML scraper using `golang.org/x/net/html` token visitor; targets the **second** `<table class="table">`
-- [ ] **M4.6** `internal/io/cache` — FileCache with `modifiedTime` gating + two TTL knobs + atomic writes (`os.Rename`)
-- [ ] **M4.7** `services/banksync.SyncToSheets` + `fuj sync` subcommand
-- [ ] **M4.8** `services/banksync.InferPayments` + `fuj infer [--dry-run]` subcommand
+- [x] **M4.1** Design IO interfaces (`SheetsClient`, `DriveClient`, `FioClient`, `FileCache`) + in-memory fakes seeded from M3 fixtures
+- [x] **M4.2** `internal/io/sheets` — Google client (read + append + batchUpdate); fake with call-capture
+- [x] **M4.3** `internal/io/drive` — Drive `modifiedTime` client + fake
+- [x] **M4.4** `internal/io/fio` — API JSON impl (token-based); parses by hardcoded `column0..column22` indices matching [fio_utils.py](scripts/fio_utils.py)
+- [x] **M4.5** `internal/io/fio` — transparent-page HTML scraper using `golang.org/x/net/html` token visitor; targets the **second** `<table class="table">`
+- [x] **M4.6** `internal/io/cache` — FileCache with `modifiedTime` gating + two TTL knobs + atomic writes (`os.Rename`)
+- [x] **M4.7** `services/banksync.SyncToSheets` + `fuj sync` subcommand
+- [x] **M4.8** `services/banksync.InferPayments` + `fuj infer [--dry-run]` subcommand; `NewSources` wires all IO into fees+reconcile
 
-**Gate:** `go test -tags=integration ./internal/io/...` round-trips against test sheet; default-tag tests run on fakes.
+**Gate:** ✅ Fakes-only unit tests; `make go-test` + `make go-lint` both green. Live smoke test deferred to first real sync run.
 
 ---
 
@@ -155,4 +155,5 @@ Goal: Go is the one true backend.
 (Add entries as you go. Format: `YYYY-MM-DD — short note`.)
 
 - 2026-05-04 — Plan approved. Versioning policy: latest stable for Go and all libs at the time M1 starts. Frontends explicitly allowed to diverge between Python and Go; only the JSON API contract is parity-locked. No reverse proxy — both backends run on different ports via `make web-py` / `make web-go`.
+- 2026-05-07 — M4 complete. Chose fakes-only unit tests (no live integration tests) and CSV-via-public-URL for attendance (no Sheets API auth required for read-only). golangci-lint gofumpt extra-rules differ slightly from standalone gofumpt; used `golangci-lint run --fix --enable-only gofumpt` to auto-resolve formatting.
 - 2026-05-04 — M1 complete. Dockerfile base changed from `distroless/static:nonroot` → `alpine:3` for debuggability (can tighten later). CLI dispatcher uses stdlib `flag`; module path `fuj-management/go`. golangci-lint v1 embedded gofumpt merges all imports into one group (no stdlib/local split) — accepted as the project style.