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>

go/cmd/parity/main.go

@@ -21,7 +21,7 @@ var allRoutes = []string{"/api/adults", "/api/juniors", "/api/payments"}
 
 // defaultAllowlist holds dotted key paths to strip before diffing.
 // render_time.total is forward-compatible insurance: today it lives in the Jinja
-// template context only (app.py inject_render_time) and is logged via
+// template context only (legacy/app.py inject_render_time) and is logged via
 // middleware/timer.go on the Go side, so it isn't in any JSON response. If either
 // side ever surfaces it under a render_time envelope, the scrubber handles it.
 var defaultAllowlist = []string{"render_time.total"}