Server: clean up abandoned blobs after PQ migration

## Background

As part of the PQ sync-auth migration (asymmetric key bundle replacing the AES sync key), each device that migrates generates a new blob path and pushes to it, leaving its old `${syncToken}.enc` blob in place on the server. The old blob must remain reachable during the migration window so that other devices can do a final pull before re-keying.

Once all devices have migrated, the old blobs are permanently unreachable — no client holds the old syncToken anymore, and the old auth protocol has no way to claim them. They will accumulate on disk and count against the server's quota.

## What needs to be decided

The definition of "abandoned" is still to be determined. Candidates:

- **Age-based:** blobs not accessed (GET or PUT) for N days are considered abandoned and eligible for eviction.
- **Format-based:** blobs that don't start with the `0x02` version byte (i.e. old AES-GCM format) are legacy and can be evicted after a grace period.
- **Auth-based:** blobs whose syncToken no longer maps to any active session and haven't been accessed in N days.
- **Combination:** old-format blobs that haven't been accessed in N days (conservative version of format-based).

## What needs to be built

- A server-side cleanup job (cron or on-demand) that evicts abandoned blobs.
- An operator-configurable grace period (e.g. `SYNC_BLOB_ABANDONED_DAYS=90`).
- Logging/metrics so operators can see how many blobs were evicted and why.
- Documentation in `docs/DEPLOYMENT.md` covering the cleanup behaviour and how to tune it.

## Notes

- The existing LRU eviction in `sync-server/handlers/blob.ts` handles quota pressure but is not the right tool here — it evicts the oldest blobs globally, not specifically abandoned ones.
- This issue is a follow-up to the PQ sync-auth redesign. It can be addressed after the migration is shipped and operational experience informs what "abandoned" should mean in practice.