Convert to Flask app with password-protected admin panel

Links now live in data/links.json instead of hardcoded HTML, editable
via /admin (add/rename/reorder/delete sections and links, per-link
color and half-width side-by-side layout). New pages can be added
without code changes. Docker image only needs rebuilding on code
changes now, not on content changes (data/ is volume-mounted).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,3 @@
+ADMIN_PASSWORD=change-me
+SECRET_KEY=change-me-to-a-long-random-string
+COOKIE_SECURE=true