feat: add admin report callback for join requests

[TheOnlyOne001]

Summary

• Add a Report inline button to join-request challenge messages
• Verify the tapping user is a group admin with getChatMember before accepting a report
• Persist each report as an append-only Cloudflare KV record keyed by chat, reported user, timestamp, and reporter for future export
• Answer callback queries for success, non-admin taps, and admin verification failures

Notes

Telegram inline keyboards are rendered the same way for every viewer in a group, so the bot cannot make a button visually appear only for admins. This PR enforces the admin-only requirement at the bot/API boundary before writing anything to KV.

Fixes #26

Validation

• cargo fmt --all -- --check
• cargo check
• cargo test
• cargo check --target wasm32-unknown-unknown
• git diff --check