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>

flake.nix

@@ -14,7 +14,8 @@
         python = pkgs.python3;
 
         pythonEnv = python.withPackages (ps: with ps; [
-          python-telegram-bot
+          # python-telegram-bot was used by the now-deleted bot.py polling
+          # loop; the Mini App backend doesn't need it. Kept off this list.
           python-dotenv
           aiohttp
           pytest
@@ -27,8 +28,7 @@
           packages = [ pythonEnv pkgs.cloudflared ];
           shellHook = ''
             echo "💪 BigBiggerBiggestBot dev shell"
-            echo "   Run:  python start.py    (server + tunnel + bot)"
-            echo "   Run:  python bot.py       (bot only, no mini app)"
+            echo "   Run:  python start.py    (server + tunnel)"
           '';
         };