refactor(consistency): blobs_consistency reads only the database

`blobs_consistency` probed `blob_exists` once per row and, under
`?deep=true`, read and re-hashed every blob. `backend_consistency`
already reports the same `blob_missing_from_backend` from its
merge-join — so the probe was duplicated work that found strictly less
(a DB walk cannot see backend-only orphans by construction) at N round
-trips instead of one enumeration. Every scheduled sweep paid for it.

All three physical checks move to `backend_consistency`:

* `blob_missing_from_backend` was already there; the duplicate is gone.
* `blob_corrupted` / `blob_unreadable` hook the matched arm of the
  merge-join, which holds exactly the key pairs worth reading. Guarded
  by `in_range` so a pair past the horizon is not read twice, and
  `params.deep` is persisted on a fresh run and read back on resume so
  a paused deep scan does not silently continue shallow.

Deep mode belongs there because it is backend work end to end: the
only DB input is the hash. Keeping it in `blobs_consistency` forced
that tenant to carry a backend for one flag.

What remains is the half that needs no backend: `refcount_mismatch`
and its repair. The constructor drops from five parameters to two —
no backend, no storage_entries, no storage_path_fallback — and
`?storage=<name>` / `?deep=true` are now inert there, which the
job description says outright.

`affected_files` is needed by both tenants, so it moves to a shared
`blob_diagnostics` module rather than being copied.
`PROBED_STORAGE_PARAM` moves to `backend_consistency`: it was defined
in `blobs_consistency` and re-exported, which is backwards once the
DB-only tenant has no entry to scope. The create-grace window goes
with the probe — it existed to avoid flagging a blob whose bytes had
landed before its row, and the refcount comparison reads one
consistent snapshot.

Known cost: `backend_consistency` returns `backend_unenumerable` on
Azure and mid-migration, so on those configs missing bytes now go
unreported where the per-row probe caught them. That argues for the
Azure enumeration impl, not for keeping the probe.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/plan/storage-key-rotation.md

@@ -179,7 +179,17 @@ Guardrail:
   head pair's fingerprint differs from the second entry's — signals "you
   added new keys but haven't rotated legacy blobs yet".
 * A **legacy-blob counter** is surfaced in the admin panel per storage entry.
-  The counter is maintained by `blobs_consistency`: during its normal walk it
+
+  > **Retarget (post-split).** This plan names `blobs_consistency` as the
+  > host for the magic-byte branch throughout. That is no longer the right
+  > tenant: `blobs_consistency` became database-only and opens no backend,
+  > while `backend_consistency` owns every physical check and already holds
+  > the enumeration. Read `backend_consistency` wherever the sections below
+  > say `blobs_consistency`. Nothing else about the design changes — the
+  > magic-byte check still rides along on an existing walk, and still lands
+  > in the run's `stats` bag.
+
+  The counter is maintained by that scan: during its normal walk it
   branches on the magic-byte check and records the legacy count as a run
   statistic on `jobs.recoverable_runs` (existing surface, no schema hit). The
   admin panel reads the most recent count and displays it. Refresh cadence is