refactor: move Python implementation to legacy/, make Go primary

Go (go/) becomes the primary implementation. The original Python
version moves to legacy/ and is kept runnable; new features are still
built in both (see the Dual implementation rule in CLAUDE.md).

- git mv app.py, scripts/, templates/, tests/, pyproject.toml, uv.lock
  and build/ into legacy/
- Makefile: Python targets point at legacy/, venv in legacy/.venv,
  CREDENTIALS_PATH exported for the root .secret/ file
- CI and make image build the Python image with legacy/ as context;
  the Makefile is no longer copied into the image
- legacy/.dockerignore moved to the context root so it takes effect
- CLAUDE.md, docs/README.md, Go comments, fixture README, capture
  script and VS Code launch config updated to the new layout

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Makefile

@@ -1,26 +1,30 @@
 .PHONY: help fees match web web-py web-debug web-go go-build go-test go-test-all go-parity go-run go-sync-debug go-lint capture-fixtures parity 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
+# Legacy Python implementation lives in legacy/ (Go in go/ is primary).
+LEGACY := legacy
+export PYTHONPATH := $(CURDIR)/$(LEGACY)/scripts:$(CURDIR)/$(LEGACY):$(PYTHONPATH)
+VENV := $(LEGACY)/.venv
+PYTHON := $(CURDIR)/$(VENV)/bin/python3
+CREDENTIALS := $(CURDIR)/.secret/fuj-management-bot-credentials.json
+# Both implementations read CREDENTIALS_PATH; the Python default would resolve under legacy/.
+export CREDENTIALS_PATH ?= $(CREDENTIALS)
 
 GO_SRC := go
 GO_BIN := bin/fuj
 
-$(PYTHON): .venv/.last_sync
+$(PYTHON): $(VENV)/.last_sync
 
-.venv/.last_sync: pyproject.toml
-	uv sync
-	touch .venv/.last_sync
+$(VENV)/.last_sync: $(LEGACY)/pyproject.toml
+	uv sync --project $(LEGACY)
+	touch $(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 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         - Start legacy Python dashboard (alias for web-py)"
+	@echo "  make web-py      - Start legacy Python dashboard on :5001"
+	@echo "  make web-go      - Build and start Go dashboard on :8080 (primary)"
 	@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 unit tests"
@@ -44,21 +48,21 @@ help:
 	@echo "  make docs        - Serve documentation in a browser"
 
 venv:
-	uv sync
+	uv sync --project $(LEGACY)
 
 fees: $(PYTHON)
-	$(PYTHON) scripts/calculate_fees.py
+	$(PYTHON) $(LEGACY)/scripts/calculate_fees.py
 
 match: $(PYTHON)
-	$(PYTHON) scripts/match_payments.py
+	$(PYTHON) $(LEGACY)/scripts/match_payments.py
 
 web: web-py
 
 web-py: $(PYTHON)
-	$(PYTHON) app.py
+	$(PYTHON) $(LEGACY)/app.py
 
 web-debug: $(PYTHON)
-	FLASK_DEBUG=1 $(PYTHON) app.py
+	FLASK_DEBUG=1 $(PYTHON) $(LEGACY)/app.py
 
 go-build:
 	cd $(GO_SRC) && go build -trimpath \
@@ -82,10 +86,10 @@ capture-fixtures: $(PYTHON)
 	              infer_transaction_details format_date reconcile; do \
 	    dir="go/tests/fixtures/$$([[ $$func == reconcile ]] && echo reconcile || echo pure/$$func)"; \
 	    mkdir -p "$$dir"; \
-	    PYTHONPATH=scripts:. $(PYTHON) scripts/capture_fixtures.py --func $$func --all \
+	    $(PYTHON) $(LEGACY)/scripts/capture_fixtures.py --func $$func --all \
 	    | while IFS= read -r line; do \
 	        case_id=$$(echo "$$line" | $(PYTHON) -c "import sys,json; print(json.load(sys.stdin)['case'])"); \
-	        echo "$$line" | $(PYTHON) scripts/scrub_fixtures.py > "$$dir/$${case_id}.json"; \
+	        echo "$$line" | $(PYTHON) $(LEGACY)/scripts/scrub_fixtures.py > "$$dir/$${case_id}.json"; \
 	    done; \
 	    echo "  $$func done"; \
 	done
@@ -112,34 +116,34 @@ image:
 		--build-arg GIT_TAG=$$(git describe --tags --always 2>/dev/null || echo "untagged") \
 		--build-arg GIT_COMMIT=$$(git rev-parse --short HEAD) \
 		--build-arg BUILD_DATE=$$(date -u +%Y-%m-%dT%H:%M:%SZ) \
-		-f build/Dockerfile .
+		-f $(LEGACY)/build/Dockerfile $(LEGACY)
 
 run:
 	docker run -it --rm -p 5001:5001 -v $(CURDIR)/.secret:/app/.secret:ro fuj-management:latest
 
 sync: $(PYTHON)
-	$(PYTHON) scripts/sync_fio_to_sheets.py --credentials .secret/fuj-management-bot-credentials.json
+	$(PYTHON) $(LEGACY)/scripts/sync_fio_to_sheets.py --credentials $(CREDENTIALS)
 
 sync-2025: $(PYTHON)
-	$(PYTHON) scripts/sync_fio_to_sheets.py --credentials .secret/fuj-management-bot-credentials.json --from 2025-10-01 --to 2025-12-31 --sort-by-date
+	$(PYTHON) $(LEGACY)/scripts/sync_fio_to_sheets.py --credentials $(CREDENTIALS) --from 2025-10-01 --to 2025-12-31 --sort-by-date
 
 sync-2026: $(PYTHON)
-	$(PYTHON) scripts/sync_fio_to_sheets.py --credentials .secret/fuj-management-bot-credentials.json --from 2026-01-01 --to 2026-12-31 --sort-by-date
+	$(PYTHON) $(LEGACY)/scripts/sync_fio_to_sheets.py --credentials $(CREDENTIALS) --from 2026-01-01 --to 2026-12-31 --sort-by-date
 
 sync-debug: $(PYTHON)  ## Dry-run Python sync with Fio diagnostics and txn table (default DAYS=30)
-	$(PYTHON) scripts/sync_fio_to_sheets.py --credentials .secret/fuj-management-bot-credentials.json --days $(DAYS) --dry-run --print-fio-table
+	$(PYTHON) $(LEGACY)/scripts/sync_fio_to_sheets.py --credentials $(CREDENTIALS) --days $(DAYS) --dry-run --print-fio-table
 
 infer: $(PYTHON)
-	$(PYTHON) scripts/infer_payments.py --credentials $(CREDENTIALS)
+	$(PYTHON) $(LEGACY)/scripts/infer_payments.py --credentials $(CREDENTIALS)
 
-reconcile:  ## Match payments against attendance
-	export PYTHONPATH=$(PYTHONPATH):$(CURDIR)/scripts && $(PYTHON) scripts/match_payments.py --credentials $(CREDENTIALS)
+reconcile: $(PYTHON) ## Match payments against attendance
+	$(PYTHON) $(LEGACY)/scripts/match_payments.py --credentials $(CREDENTIALS)
 
 test: $(PYTHON) ## Run web application tests
-	export PYTHONPATH=$(PYTHONPATH):$(CURDIR)/scripts:$(CURDIR) && $(PYTHON) -m unittest discover tests
+	cd $(LEGACY) && $(PYTHON) -m unittest discover tests
 
 test-v: $(PYTHON) ## Run tests with verbose output
-	export PYTHONPATH=$(PYTHONPATH):$(CURDIR)/scripts:$(CURDIR) && $(PYTHON) -m unittest discover -v tests
+	cd $(LEGACY) && $(PYTHON) -m unittest discover -v tests
 
 docs: ## Serve documentation locally
 	@echo "Starting documentation server at http://localhost:8000"