feat(ecstore): add the on-demand migration backfill job (#7087)

* feat(ecstore): add on-demand migration backfill job core

Add the background backfill job for on-demand migration
(rustfs/backlog#2159): a durable checkpoint under
buckets/<bucket>/on-demand-migration-backfill.json saved by If-Match
compare-and-set every 1000 keys or 10 s, a 60 s owner lease renewed by
every save, a recovery pass that takes over expired leases (or jobs this
node owned before a restart) and cancels jobs whose config changed, and a
main loop over the source ListObjectsV2 pages with the skip_existing
policy, dry runs, bounded outstanding pulls and wait-on-full enqueueing.

The pull queue gains per-job completion reports so the job can count
pulled/failed keys (hashes only), and pull permits become two-tier so an
online miss is never queued behind a backfill pull.

* feat(admin): expose on-demand migration backfill job

Wire the ODM-12 backfill job (rustfs/backlog#2159) to its operators:
POST /v3/on-demand-migration/{bucket}/backfill?op=start|cancel and
GET .../backfill return the checkpoint document, GET .../status gains a
backfill summary, and the recovery loop plus the process-wide runner are
installed at startup. Backfill control reuses
Set/GetBucketOnDemandMigrationAction and is recorded in the route policy,
the registration matrix and the admin route snapshot.

Add the rustfs-madmin wire types and client methods with golden fixtures
shared by the server tests, the backfill_* metric descriptors and their
collector, and three e2e scenarios: a full backfill across list pages,
cancellation, and resuming from the persisted continuation token after a
server restart.

.config/e2e-full-selection.txt

@@ -1,2 +1,2 @@
-sha256-darwin=09bc29b3e0ed7c5779331740e18e1d5ca05cb240c1363d1ecc95c1ef1e337038
+sha256-darwin=efd7357af1e998134df7d01e0a10d7186898e47abbe86ae6ecb6aef1f8c58357
 sha256-linux=86e69337ad1440252a2ee20a12063c989ed12442d3b1ddf9e9233acf0f2ec089