feat(dlq): per-job-type dead-letter breakdown helper for the DLQ dashboard

[JSONbored]

countRecentDeadLetters (src/db/repositories.ts:2133) returns a single total from the github_app.dlq_dead_lettered audit events, but the planned DLQ management UI (#1208) needs the breakdown BY jobType to know what's failing. Add a repository helper countRecentDeadLettersByType(env, sinceIso): Record<string,number> that GROUP BYs the jobType stored in the audit metadata. Pure read-only aggregation; does not touch the merge/close decision path.

Deliverables

• Add countRecentDeadLettersByType in src/db/repositories.ts alongside countRecentDeadLetters, reading the jobType from the audit metadata
• Return an empty object (not null) when there are no dead letters since sinceIso
• Sort/normalize keys deterministically; unknown jobType falls back to "unknown"
• Unit tests: multiple types, single type, none (empty-object branch), and the unknown-fallback branch

References

• src/db/repositories.ts:2131
• src/db/repositories.ts:2133
• src/queue/dlq.ts:39

Part of #1208.

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size: S · gittensor:feature

[linear-code]

JSO-2199