refactor: drop the slash-command bot — Mini App is the only interface

bot.py is gone. /start, /history, /stats, /delete, /export,
/feedback, and the text-message workout parser are no longer
exposed. Everything those commands did is already available in
the Mini App (history listing, stats, edit/delete, JSON export
via /api/export/json, etc.).

Why: prod runs behind a Mini App URL, and shipyard staging is a
tenant under the existing shipyard_poc_bot which polls Telegram
itself. A second polling process on the same token would 409. By
removing polling entirely, prod and staging share one
architecture: a pure HTTP server validated against whatever
BOT_TOKEN is provided.

Changes:
- delete bot.py
- start.py: stop spawning the bot subprocess; load token, start
  server, optionally start cloudflared. WEBAPP_URL still skips
  the tunnel.
- flake.nix / requirements.txt: drop python-telegram-bot.
- README: rewrite to reflect Mini-App-only architecture.

The prod systemd unit doesn't need to change — its ExecStart is
`python start.py`, which now boots only the server (+ no tunnel
since WEBAPP_URL is set in the unit env).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

requirements.txt

@@ -1,2 +1,2 @@
-python-telegram-bot>=21.0
 python-dotenv>=1.0
+aiohttp>=3.9