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/architecture/backend-storage.md

@@ -234,27 +234,38 @@ can be safely dropped.
 
 ---
 
-## 4. Blob consistency (`blobs_consistency`)
+## 4. Blob consistency — two jobs, split by what they read
 
-Read-only recoverable job that walks `storage.blobs` and reports
-divergence between the DB registry and the physical backend.
+The registry side and the physical side are separate tenants. They
+used to be one, with `blobs_consistency` probing the backend once per
+row; that probe found strictly less than the merge-join below, at N
+round-trips instead of one enumeration, so it was removed.
 
-### Shallow mode (default)
+### `blobs_consistency` — database only
 
-Per row:
+Walks `storage.blobs` and compares `ref_count` against the reference
+count computed from `storage.files.blob_hash` +
+`chunk_manifests.chunk_hashes[]`. On mismatch: `refcount_mismatch`
+(severity `inconsistent`), repairable under `?repair=true`.
 
-- `blob_exists(hash)` on the active backend → if false, record
-  `blob_missing_from_backend` (severity `data_loss`)
-- Compare `ref_count` against the actual reference count computed
-  from `SUM` over `storage.files.blob_hash` + `chunk_manifests.chunk_hashes[]`
-  → if mismatch, record `refcount_mismatch` (severity `inconsistent`)
+It opens no backend and makes no network call. `?storage=<name>` and
+`?deep=true` are inert. Cost is one aggregate SQL per row.
 
-Cost: one existence probe + one aggregate SQL per row. Fast on
-S3/Azure (single HEAD).
+### `backend_consistency` — everything physical
 
-### Deep mode (`?deep=true`)
+Merge-joins the backend's enumeration against `storage.blobs`, both
+ordered by hash, yielding both deltas in one pass:
 
-Adds a full read of every blob:
+- bytes with no registry row → `orphan_blob` (severity `inconsistent`)
+- a registry row with no bytes → `blob_missing_from_backend`
+  (severity `data_loss`)
+
+`?storage=<name>` scopes it to any declared entry rather than the live
+backend.
+
+### Deep mode (`?deep=true`, on `backend_consistency`)
+
+For every hash present on both sides, adds a full read:
 
 - Stream the blob through `EncryptedBlobBackend::get_blob_stream`
   (strips header, decrypts if needed, applies BLAKE3 rescue for