fix(relay): stop panicking the ingest worker on reactions to project events

[Chessing234]

A NIP-25 reaction whose target is a project root or project comment (kind
1621 issue, 1618 PR, or a kind-1 comment on one) carries no h tag, so
channel_id is None on the reaction write path. The conformance-trace
emission asserted a channel was always present:

channel: channel_label(channel_id.expect("reaction path has channel")),

so the worker panicked at ingest.rs:2824. The row was inserted before the
panic, so the client saw a failed request for a persisted event and retried,
and the duplicate branch carried the same expect, head-of-line blocking a
durable publish queue forever.

Mirror the message write's three-way split at the same seam:
(Some, true) -> WriteInsert, (Some, false) -> WriteDuplicate, (None, _)
-> WriteInsertGlobal. The conformance vocabulary already models channel-less
writes; only the reaction path was missing it.

Closes #4936

[ravarora2]

🤖 Tested locally end-to-end — LGTM. ✅

Reproduced the panic on main and confirmed the fix resolves it:

Before (buggy main): published a global kind:1 note (no h tag → channel_id = NULL) and reacted to it. The ingest worker panicked:

thread 'tokio-rt-worker' panicked at crates/buzz-relay/src/handlers/ingest.rs:2849:51:
reaction path has channel

Postgres confirmed the exact failure mode described in the PR: the reaction row was persisted before the panic, so the client's retry hit the Duplicate branch and surfaced a misleading duplicate: reaction already exists.

After (this branch): same steps —

• publish global note → accepted: true
• react once → accepted: true (no panic)
• react again → accepted: false "duplicate" (correct no-op, no panic)
• panics in relay log: 0

The three-way match (channel_id, was_inserted) routing the channel-less case to WriteInsertGlobal is the right call, and it mirrors the existing message write path. The #[ignore]d regression test (test_reaction_to_channel_less_event_is_accepted) is the automated form of exactly what I ran by hand.

[ravarora2]

lgtm