feat: Go rewrite M1 — skeleton, tooling, and hello server

Stand up the Go project alongside the Python backend so both run
independently during migration. `make web-go` builds and serves on :8080;
`make web-py` (alias: `make web`) keeps the Python side on :5001.

- go/: new module `fuj-management/go` (Go 1.26)
  - cmd/fuj: stdlib-flag dispatcher; `server` + `version` work,
    fees/reconcile/sync/infer stubbed for M2/M4
  - internal/config: env loader mirroring scripts/config.py
  - internal/logging: slog setup, level taken from config
  - internal/web: net/http ServeMux + request-timer middleware
  - build/Dockerfile: golang:1.26 → alpine:3 multi-stage image
  - .golangci.yml: govet, staticcheck, errcheck, gofumpt, unused
- Makefile: web→web-py alias; go-build/go-test/go-run/go-lint/web-go
- CI: parallel build-go job in .gitea/workflows/build.yaml (<tag>-go image)
- docs/plans/: M1 kickoff plan + progress tracker (M1 complete)
- .claude/settings.json: gofumpt + golangci-lint permissions

Gate: make go-build ✓  make go-lint ✓  make go-test ✓  curl :8080 ✓

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

go/internal/config/config.go

@@ -0,0 +1,56 @@
+package config
+
+import (
+	"os"
+	"strconv"
+	"time"
+)
+
+// Google Sheets IDs — change in code if sheets change (not from env).
+const (
+	AttendanceSheetID = "1E2e_gT_K5AwSRCDLDTa2UetZTkHmBOcz0kFbBUNUNBA"
+	PaymentsSheetID   = "1Om0YPoDVCH5cV8BrNz5LG5eR5MMU05ypQC7UMN1xn_Y"
+	JuniorSheetGID    = "1213318614"
+)
+
+// Config holds all runtime configuration loaded from environment variables.
+// Mirrors scripts/config.py.
+type Config struct {
+	CredentialsPath  string
+	BankAccount      string
+	CacheTTL         time.Duration
+	CacheAPICheckTTL time.Duration
+	LogLevel         string
+	FioAPIToken      string
+	ServerAddr       string
+}
+
+// Load reads configuration from the environment, applying defaults that
+// match the Python side.
+func Load() Config {
+	return Config{
+		CredentialsPath:  env("CREDENTIALS_PATH", ".secret/fuj-management-bot-credentials.json"),
+		BankAccount:      env("BANK_ACCOUNT", "CZ8520100000002800359168"),
+		CacheTTL:         envDuration("CACHE_TTL_SECONDS", 300),
+		CacheAPICheckTTL: envDuration("CACHE_API_CHECK_TTL_SECONDS", 300),
+		LogLevel:         env("LOG_LEVEL", "INFO"),
+		FioAPIToken:      env("FIO_API_TOKEN", ""),
+		ServerAddr:       env("SERVER_ADDR", ":8080"),
+	}
+}
+
+func env(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}
+
+func envDuration(key string, defaultSeconds int) time.Duration {
+	if v := os.Getenv(key); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 {
+			return time.Duration(n) * time.Second
+		}
+	}
+	return time.Duration(defaultSeconds) * time.Second
+}