relay: SUPPORTED_NIPS omits NIP-45 (COUNT) despite an implemented handler

## What's wrong

\`crates/buzz-relay/src/nip11.rs\` advertises \`SUPPORTED_NIPS = [1,2,10,11,16,17,23,25,29,33,38,42,50,56]\` — NIP-45 is not in the list.

But \`crates/buzz-relay/src/handlers/count.rs\` implements a working NIP-45 COUNT handler (`POST /count` and the WebSocket COUNT message are both live).

## Impact

Clients that check NIP-11 capability advertisement before using COUNT will incorrectly conclude the relay doesn't support it and avoid a working feature.

## Fix

Add `45` to `SUPPORTED_NIPS` in `nip11.rs`.

## Provenance

Found during a documentation-corpus review of `launchpad/docs/corpus/platforms/relay/nip-11.md` (launchpad-26/buzz PR #1981). Not a documentation bug — the code itself under-advertises its own capability.