chore: add server-changes entry for OTLP Postgres fallback fix

Author: matt-aitken

.server-changes/otlp-postgres-store-fallback.md

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+---
+area: webapp
+type: fix
+---
+
+Restore Postgres fallback for non-ClickHouse OTLP spans. Environments where runs carry a Postgres-backed taskEventStore (taskEvent / taskEventPartitioned) were receiving HTTP 500 from the OTLP ingest endpoints because the ClickHouse factory threw an error when passed those store values. The throw aborted the entire OTLP batch in #exportEvents. Non-ClickHouse stores are now routed directly to the Postgres eventRepository (matching the existing pattern in eventRepository/index.server.ts), and the ClickHouse factory call is wrapped in a try/catch that falls back to Postgres so any future unexpected store values degrade gracefully rather than failing the whole request.