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>

Makefile

@@ -1,10 +1,13 @@
-.PHONY: help fees match web web-debug image run sync sync-2026 test test-v docs
+.PHONY: help fees match web web-py web-debug web-go go-build go-test go-run go-lint image run sync sync-2026 test test-v docs
 
 export PYTHONPATH := scripts:$(PYTHONPATH)
 VENV := .venv
 PYTHON := $(VENV)/bin/python3
 CREDENTIALS := .secret/fuj-management-bot-credentials.json
 
+GO_SRC := go
+GO_BIN := bin/fuj
+
 $(PYTHON): .venv/.last_sync
 
 .venv/.last_sync: pyproject.toml
@@ -13,20 +16,25 @@ $(PYTHON): .venv/.last_sync
 
 help:
 	@echo "Available targets:"
-	@echo "  make fees   - Calculate monthly fees from the attendance sheet"
-	@echo "  make match  - Match Fio bank payments against expected attendance fees"
-	@echo "  make web        - Start a dynamic web dashboard locally"
-	@echo "  make web-debug  - Start a dynamic web dashboard locally in debug mode"
-	@echo "  make image      - Build an OCI container image"
-	@echo "  make run    - Run the built Docker image locally"
+	@echo "  make fees        - Calculate monthly fees from the attendance sheet"
+	@echo "  make match       - Match Fio bank payments against expected attendance fees"
+	@echo "  make web         - Start Python dashboard (alias for web-py, until M8)"
+	@echo "  make web-py      - Start Python dashboard on :5001"
+	@echo "  make web-go      - Build and start Go dashboard on :8080"
+	@echo "  make web-debug   - Start Python dashboard in debug mode"
+	@echo "  make go-build    - Build Go binary to bin/fuj"
+	@echo "  make go-test     - Run Go tests"
+	@echo "  make go-lint     - Run golangci-lint on Go code"
+	@echo "  make image       - Build Python OCI container image"
+	@echo "  make run         - Run the built Python Docker image locally"
 	@echo "  make sync        - Sync Fio transactions to Google Sheets"
 	@echo "  make sync-2025   - Sync Fio transactions for Q4 2025 (Oct-Dec)"
 	@echo "  make sync-2026   - Sync Fio transactions for the whole year of 2026"
 	@echo "  make infer       - Infer payment details (Person, Purpose, Amount) in the sheet"
 	@echo "  make reconcile   - Show balance report using Google Sheets data"
 	@echo "  make venv        - Sync virtual environment with pyproject.toml"
-	@echo "  make test        - Run web application infrastructure tests"
-	@echo "  make test-v      - Run tests with verbose output"
+	@echo "  make test        - Run Python web application infrastructure tests"
+	@echo "  make test-v      - Run Python tests with verbose output"
 	@echo "  make docs        - Serve documentation in a browser"
 
 venv:
@@ -38,12 +46,33 @@ fees: $(PYTHON)
 match: $(PYTHON)
 	$(PYTHON) scripts/match_payments.py
 
-web: $(PYTHON)
+web: web-py
+
+web-py: $(PYTHON)
 	$(PYTHON) app.py
 
 web-debug: $(PYTHON)
 	FLASK_DEBUG=1 $(PYTHON) app.py
 
+go-build:
+	cd $(GO_SRC) && go build -trimpath \
+	  -ldflags "-X main.version=$$(git describe --tags --always 2>/dev/null || echo dev) \
+	            -X main.commit=$$(git rev-parse --short HEAD) \
+	            -X main.buildDate=$$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+	  -o ../$(GO_BIN) ./cmd/fuj
+
+go-test:
+	cd $(GO_SRC) && go test -race ./...
+
+go-run: go-build
+	./$(GO_BIN) $(ARGS)
+
+go-lint:
+	cd $(GO_SRC) && golangci-lint run ./...
+
+web-go: go-build
+	./$(GO_BIN) server
+
 image:
 	docker build -t fuj-management:latest \
 		--build-arg GIT_TAG=$$(git describe --tags --always 2>/dev/null || echo "untagged") \