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>

.gitignore

@@ -4,3 +4,6 @@
 
 # local tmp folder
 tmp/
+
+# go build output
+bin/