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>

docs/README.md

@@ -4,6 +4,8 @@ Welcome to the documentation for the FUJ Management application.
 
 This project automates financial and operational management for the FUJ (Frisbee Ultimate Jablonec) club.
 
+> **Go is the primary implementation** (`go/`). The project was originally developed in Python; that version now lives in `legacy/` and is considered legacy. New features are still built in both. Most guides below predate this change and describe the Python version with its old paths (`app.py`, `scripts/` are now `legacy/app.py`, `legacy/scripts/`).
+
 ## Navigation
 
 Use the sidebar to explore the documentation: