datastores: getLatestForumPostDates degrades to empty map on query error — lastForumPostDate silently blank on 200s

[SyniRon]

Found during the wave-2 silent-failure review (pre-existing, shared by both stacks).

datastores/mysql.go:599 (getLatestForumPostDates): on query error it logs and returns an empty map — every lastForumPostDate in a roster/search response silently degrades to "" on a 200 during a partial outage. Logged at least, but the caller cannot distinguish degraded from genuinely-empty.

Fix shape: propagate the error so the route's outage path answers (or return a sentinel the caller converts). Needs the same parity-ruling caveat as the sibling ticket: the current degrade shape is what production has always served.

[SyniRon]

This was generated by AI during triage.

Maintainer ruling (2026-06-06, recorded during the pre-#131 triage): the outage shape is ruled parity — the new stack keeps the empty-map degrade (every lastForumPostDate blank on a 200 during partial outage), exactly what prod has always served. Error propagation to the route's outage path will not land.

The fix rides #134 as an observability change: the query error stays logged and additionally becomes meterable, without altering the degrade shape consumers see.

Labels: ready-for-agent (the ruling resolves the open design question) + blocked (sequenced into #134's wave).