fix: harden API, add rate limiting, disable debug by default

- Add input validation and error handling across all API endpoints
  (malformed JSON, missing fields, invalid types return proper 4xx)
- Add per-IP rate limiting (10/min new games, 60/min other requests)
- Add session rollback on unhandled exceptions
- Default debug=False, add --debug CLI flag to opt in
- Frontend: add fetch error handling, fix feedback timeout stacking,
  disable buttons during requests, allow clicking found cells for
  overlapping word selection
- Expand seed data to 7 categories with 25-35 words each

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

templates/about.html

@@ -1,8 +1,45 @@
 {% extends "base.html" %}
 
+{% block active_page %}{% set active_page = "about" %}{% end %}
+
 {% block title %}Word Search — About{% end %}
 
 {% block content %}
-<h2>About</h2>
-<p>About page coming soon.</p>
+<h2 class="mb-4">ℹ️ About</h2>
+
+<div class="content-section">
+    <h3>🧩 What is Word Search?</h3>
+    <p>
+        Word Search is a browser-based puzzle game where you find hidden words in a
+        grid of letters. Words can be placed horizontally, vertically, or
+        diagonally — forwards or backwards. The backend generates puzzles,
+        validates your selections, and tracks your progress in real time.
+    </p>
+</div>
+
+<div class="content-section">
+    <h3>🛠️ Built With</h3>
+    <ul>
+        <li><strong>Python</strong> — core language</li>
+        <li><strong>Tornado</strong> — async web framework</li>
+        <li><strong>Bootstrap 5</strong> — responsive UI</li>
+        <li><strong>Bootstrap Icons</strong> — interface icons</li>
+    </ul>
+</div>
+
+<div class="content-section">
+    <h3>✍️ Authors</h3>
+    <ul>
+        <li><strong>Mathew Guest</strong> — creator and developer</li>
+        <li><strong>🤖 Claude</strong> (Anthropic) — AI co-author</li>
+    </ul>
+</div>
+
+<div class="content-section">
+    <h3>📂 Source Code</h3>
+    <p>
+        The source code for this project will be available on GitHub.
+        <em>Link coming soon.</em>
+    </p>
+</div>
 {% end %}