feat(jobs): jobs describe themselves — description, mutates, repair_description

The admin panel had no repair toggle wired to anything but a hardcoded
name list naming the two refcount tenants, so `thumb_derived_import` and
`thumb_attached_import` could not be run in repair mode from the UI at
all despite supporting it. And nothing in the job list said what any
given job does or whether clicking Run on production writes anything.

Three defaulted methods on `JobHandler` and `RecoverableJobHandler`:

    fn description(&self) -> &'static str
    fn mutates(&self) -> Mutates          // Never | Always | OnRepairOnly
    fn repair_description(&self) -> Option<&'static str>

`RecoverableAdapter` forwards them — the registry only holds
`dyn JobHandler`, so a tenant's metadata is invisible otherwise, and
falling back to the defaults would report every recoverable job as
read-only, including the ones that delete files.

Three values rather than a boolean because a job can be read-only by
default and destructive under `?repair=true`; a boolean answers wrongly
for one of its two modes, and `false` on something that unlinks files is
the dangerous direction to be wrong in. `repair_description` returning
`Option` collapses "does it repair" and "what does repair do" into one
method: presence gates the toggle, content is the confirmation text —
which the frontend cannot invent, since correcting a counter and
deleting sidecars are not the same warning.

`OnRepairOnly` with no `repair_description` is rejected at registration:
it claims to mutate only under a flag it does not support.

All 17 registered jobs declare all three. The panel now renders the
description under each name, badges read-only jobs, confirms before a
plain run of a mutating one, and offers the repair variant off the
backend flag instead of the name list.

Descriptions are English in the trait, next to the behaviour: one in
`locales/*.json` rots invisibly the moment a job changes, and a
translator cannot know what `manifests_consistency` reconciles. i18n can
layer on later keyed by job name with these as the fallback.

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

src/infrastructure/services/backend_migration_service.rs

@@ -59,8 +59,8 @@ use crate::application::ports::blob_storage_ports::BlobStorageBackend;
 use crate::common::config::NamedStorageEntry;
 use crate::common::errors::DomainError;
 use crate::infrastructure::scheduler::{
-    JobRegistry, JobRunArgs, JobStore, JobStoreProvider, RecoverableJobHandler, RunOutcome,
-    RunStatus, record_or_log,
+    JobRegistry, JobRunArgs, JobStore, JobStoreProvider, Mutates, RecoverableJobHandler,
+    RunOutcome, RunStatus, record_or_log,
 };
 use crate::infrastructure::services::encrypted_blob_backend::{EncryptedBlobBackend, HeadCheck};
 use crate::infrastructure::services::entry_backend::{
@@ -188,6 +188,20 @@ impl RecoverableJobHandler for BackendMigrationService {
         BACKEND_MIGRATION_JOB_NAME
     }
 
+    fn description(&self) -> &'static str {
+        "Copies every blob payload from the backend the server booted with \
+         to the one the current storage settings describe. Covers legacy \
+         whole-file blobs and CDC chunks in a single walk. Resumable — a \
+         paused or crashed run continues from its cursor rather than \
+         restarting."
+    }
+
+    /// Writes bytes to the target backend. Source bytes are left in place —
+    /// the copy is additive, so an aborted migration loses nothing.
+    fn mutates(&self) -> Mutates {
+        Mutates::Always
+    }
+
     /// Definitive count — one row per blob. `SELECT COUNT(*) FROM
     /// storage.blobs` on a modern PG is a sub-second index-only scan
     /// even at millions of rows.