feat(archive): export cold monthly partitions

[KurodaKayn]

Feature Description

• Adds a PostgreSQL-only cold monthly partition archive path for managed event/session history tables.
• Exports whole cold partitions as JSONL to the configured R2/S3 object storage prefix before detaching and dropping them.
• Keeps the existing row-level archive path for partial-month retention cleanup and non-PostgreSQL runs.
• Updates the Phase 4 database optimization checklist to mark cold partition export complete while leaving archive recovery as follow-up.

Implementation Approach

• Introduces monthly partition archive specs for publish events, extension execution events, project/workspace activities, and remote browser sessions.
• Uses the PostgreSQL partition catalog plus managed partition-name bounds to select only whole partitions older than the retention cutoff.
• Deletes extension execution event claims before dropping archived extension-event partitions, and skips remote browser session partitions containing non-terminal rows.

Testing

• cd backend && go test ./... passed.
• cd backend && golangci-lint run passed with 0 issues.

Risks

• The partition path detaches and drops partitions only after upload succeeds, but production rollout should still verify object-storage retention and restore procedure before enabling aggressive retention windows.