fix: correct marquee scroll speed and add code TODOs

- Fixed scrolling marquee to use a fixed px/s speed via JS instead of
  a fixed duration, preventing it from speeding up as entries are added
- Added inline TODO comments throughout codebase to track known issues
  (rate limiting, CSRF, unbounded queries, deprecated Flask decorator,
  PII logging, schema versioning, Docker non-root user, etc.)
- Added todo-to-issue GitHub Action to auto-create Issues from TODOs on push to main
- Added .claude/ to .gitignore

scripts/guestbook_export.py

@@ -1,7 +1,8 @@
 import csv
 import sqlite3
 
-# Update the database file path if needed.
+# TODO: Hardcoded relative path — breaks if script is run from a different directory.
+# Replace with: DATABASE = os.environ.get('DATABASE_PATH', 'guestbook.db') and import os.
 DATABASE = 'guestbook.db'
 EXPORT_FILE = 'mailchimp_export.csv'