Support NIP-89 discovery for DVM job requests (kind-31989 / kind-31990) #736

Follow-up to #639. The relay can now ingest and dispatch NIP-90 job requests (#639, #727, #729, #731), but there's no support yet for NIP-89 service discovery how clients find out which DVMs a relay's workers can talk to.

What needs to be done

• Accept and serve kind-31989 (recommendation) events passive store/serve, authored externally by users. Should mostly fall out of the existing ParameterizedReplaceableEventStrategy since both kinds sit in the 30000-39999 parameterized-replaceable range needs verification + tests, not necessarily new handler code.
• Accept and serve kind-31990 (handler-info) events the same way passive accept/serve only. The relay does not author its own kind-31990 on behalf of locally-registered workers, and there is no heartbeat/withdraw-on-crash publishing subsystem for this (confirmed out of scope for now).
• Tests covering create/replace/serve for both kinds via REQ filters.