fix(relay): set received_at in the system-channel fan-out test helper (#9214)

## Summary

`buzz-relay` lib tests fail to compile on `main`: `error[E0063]: missing
field received_at in initializer of buzz_pubsub::ChannelEvent` at
`crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs:1549`.

This is a semantic merge conflict. #9195 added
`ChannelEvent::received_at`, and #8519 merged a minute later with a test
helper that builds a `ChannelEvent` without it. Neither PR's CI saw the
other. This sets the field to `tokio::time::Instant::now()`, as every
other initializer does.

It breaks clippy (`Rust Lint`) and the backend-integration test archive
for every open PR, because CI builds the merge with `main`.

### Testing

- `cargo fmt --check` and `cargo clippy -p buzz-relay --all-targets -D
warnings` on a cloud workstation at this head.

## What got simpler

Nothing; a one-field compile fix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Luke Tornquist <tornquist@squareup.com>
Co-authored-by: Elrond <28d6302a099e5225b02c4155ac4236e4912603df2ab08dbfc2f4fef08ce598c8@buzz.block.builderlab.xyz>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs

@@ -1550,6 +1550,7 @@ async fn fan_out_from_another_pod(f: &Fixture, channel: Uuid, name: &str) -> Str
             community_id: f.tenant.community(),
             topic: buzz_pubsub::EventTopic::Channel(channel),
             event,
+            received_at: tokio::time::Instant::now(),
         },
     )
     .await;