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/config/admin-settings.md

@@ -100,12 +100,16 @@ This one-shot repair command re-runs the same env-parse the server does at boot,
 
 ### Auditing entries other than the active one
 
-`blobs_consistency` and `backend_consistency` (recoverable jobs on the Jobs tab) accept `?storage=<name>` to probe any declared entry — not just the live one. Use this to verify a migration target before cutover, or to audit an old backend after cutover but before decommissioning:
+`backend_consistency` (a recoverable job on the Jobs tab) accepts `?storage=<name>` to audit any declared entry — not just the live one. Use this to verify a migration target before cutover, or to audit an old backend after cutover but before decommissioning:
 
 ```
-POST /api/admin/jobs/blobs_consistency/trigger?storage=<name>
+POST /api/admin/jobs/backend_consistency/trigger?storage=<name>
 ```
 
+Add `?deep=true` to also read every blob back and re-hash it, which catches silent bit-rot. That is a full read of the entry and can take hours.
+
+`blobs_consistency` does *not* accept `?storage=<name>`: it only reads the database, so there is no entry for it to scope.
+
 Unknown names 400 at the HTTP layer.
 
 ## Data Storage