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>

legacy/templates/flush-cache.html

@@ -0,0 +1,163 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>FUJ - Flush Cache</title>
+    <style>
+        body {
+            font-family: ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace;
+            background-color: #0c0c0c;
+            color: #cccccc;
+            padding: 10px;
+            margin: 0;
+            display: flex;
+            flex-direction: column;
+            align-items: center;
+            font-size: 11px;
+            line-height: 1.2;
+        }
+
+        h1 {
+            color: #00ff00;
+            font-family: inherit;
+            margin-top: 10px;
+            margin-bottom: 20px;
+            text-transform: uppercase;
+            letter-spacing: 1px;
+            font-size: 14px;
+        }
+
+        .nav {
+            margin-bottom: 20px;
+            font-size: 12px;
+            color: #555;
+            display: flex;
+            flex-direction: column;
+            gap: 10px;
+            align-items: center;
+        }
+
+        .nav > div {
+            display: flex;
+            gap: 15px;
+            align-items: center;
+        }
+
+        .nav a {
+            color: #00ff00;
+            text-decoration: none;
+            padding: 2px 8px;
+            border: 1px solid #333;
+        }
+
+        .nav a.active {
+            color: #000;
+            background-color: #00ff00;
+            border-color: #00ff00;
+        }
+
+        .nav a:hover {
+            color: #fff;
+            border-color: #555;
+        }
+
+        .nav-archived a {
+            font-size: 10px;
+            color: #666;
+            border-color: #222;
+        }
+
+        .nav-archived a.active {
+            color: #ccc;
+            background-color: #333;
+            border-color: #555;
+        }
+
+        .nav-archived a:hover {
+            color: #999;
+            border-color: #444;
+        }
+
+        .flush-container {
+            background-color: #111;
+            border: 1px solid #333;
+            padding: 30px;
+            width: 100%;
+            max-width: 600px;
+            margin-bottom: 30px;
+            text-align: center;
+        }
+
+        .flush-btn {
+            font-family: inherit;
+            font-size: 12px;
+            color: #00ff00;
+            background-color: #1a1a1a;
+            border: 1px solid #00ff00;
+            padding: 8px 24px;
+            cursor: pointer;
+            text-transform: uppercase;
+            letter-spacing: 1px;
+        }
+
+        .flush-btn:hover {
+            background-color: #00ff00;
+            color: #000;
+        }
+
+        .status {
+            margin-bottom: 15px;
+            font-size: 12px;
+        }
+
+        .status-ok { color: #00ff00; }
+
+        .footer {
+            text-align: center;
+            color: #333;
+            margin-top: 20px;
+            font-size: 10px;
+        }
+    </style>
+</head>
+
+<body>
+    <div class="nav">
+        <div>
+            <a href="/adults">[Adults]</a>
+            <a href="/juniors">[Juniors]</a>
+        </div>
+        <div class="nav-archived">
+            <span style="color: #666; margin-right: 5px;">Archived:</span>
+            <a href="/payments">[Payments Ledger]</a>
+        </div>
+        <div class="nav-archived">
+            <span style="color: #666; margin-right: 5px;">Tools:</span>
+            <a href="/sync-bank">[Sync Bank Data]</a>
+            <a href="/flush-cache" class="active">[Flush Cache]</a>
+        </div>
+    </div>
+
+    <h1>Flush Cache</h1>
+
+    {% if flushed %}
+    <div class="status">
+        <span class="status-ok">Cache flushed successfully. {{ deleted }} file(s) deleted.</span>
+    </div>
+    {% endif %}
+
+    <div class="flush-container">
+        <p style="margin-bottom: 20px; color: #888;">Clears all cached Google Sheets data and resets refresh timers.</p>
+        <form method="POST" action="/flush-cache">
+            <button type="submit" class="flush-btn">[Flush Cache]</button>
+        </form>
+    </div>
+
+    <div class="footer">
+        {{ build_meta.tag }} | {{ build_meta.commit }} | {{ build_meta.build_date }}
+    </div>
+</body>
+
+</html>