feat(transcode): the local cache disables itself, and drains at boot

Completes the pattern the thumbnail migration established, for
`.transcoded/`.

`initialize` no longer creates the tree. Creating it at boot is exactly
what kept `.thumbnails/` alive across restarts — the import removed it,
the next boot put it back, and the absence the read path gates on was
unreachable by construction. The write path already calls
`create_dir_all` on the parent before writing, so eager creation
achieved nothing except defeating the drain.

It now probes instead: one `stat`, cached for the process lifetime, and
the local-cache reads short-circuit on a relaxed atomic load when the
tree is gone. Fails open, so a service built without `initialize`
behaves as before.

One difference from the thumbnail tiers, and it is not a stalled
migration: callers with no content hash — external mounts — cannot use
the content-keyed tier at all, so they still read and write here. On an
install without such mounts the directory drains once and stays gone;
on one with them it persists, correctly.

`transcode_import?repair=true` joins the startup defaults on the same
terms as the thumbnail imports, and with the weakest safety argument
needed of the three: a transcode is a pure function of its source, so
anything deleted in error is recomputed on the next request. The
`default_startup_jobs` test failed on the change rather than being
updated silently, which is what it is for.

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

docs/config/env.md

@@ -18,7 +18,7 @@ Most runtime variables use the `OXICLOUD_` prefix. A few build-time or allocator
 | `OXICLOUD_CHUNK_DIR` | `{STORAGE_PATH}/.uploads` | Root directory for chunked-upload sessions (REST + NextCloud). Direct (non-chunked) uploads stream straight into the blob store and need no spool directory. Placement guidance: see [Storage Fine Tuning](./storage-fine-tuning.md). |
 | `OXICLOUD_REUSE_PORT` | `false` | Enable `SO_REUSEPORT` so multiple processes can share the same port. **Disabled by default** — a second accidental instance will fail with "address already in use". Enable only for deliberate multi-worker setups (process supervisor, rolling restart). Not supported on Windows. |
 | `OXICLOUD_METRICS_LISTEN` | (unset) | Prometheus `/metrics` listener address (e.g. `127.0.0.1:9090`, IPv6 allowed as `[::1]:9090`). **Unset = disabled**: no `/metrics` endpoint is bound and no metrics recorder is installed (zero runtime cost). When set, a separate HTTP listener on this address serves the text-format scrape. **Deliberately NOT merged into the main API** — no auth, CSRF, or DPoP layer in front. Bind to loopback or a private interface unless you intend to expose metrics publicly. Starter counters: `oxicloud_dpop_verify_failed_total{reason}`, `oxicloud_dpop_proof_missing_total`, `oxicloud_dpop_header_missing_on_bound_session_total`, `oxicloud_dpop_replay_detected_total`, `oxicloud_dpop_nonce_challenges_issued_total`. |
-| `OXICLOUD_STARTUP_JOBS` | `thumb_derived_import?repair=true,thumb_attached_import?repair=true` | Background jobs dispatched once at boot, comma-separated, each `name` or `name?flag=true` using the same syntax as `POST /api/admin/jobs/{name}/trigger`. Flags: `force`, `deep`, `repair`, `storage`. **The default migrates thumbnails out of the legacy `.thumbnails/` directory and deletes the originals**, so the migration completes without anyone triggering it from the admin panel; each sidecar is read back through the normal stack before it is unlinked, and every deletion is audited. An explicit value **replaces** the default; set it empty (`OXICLOUD_STARTUP_JOBS=`) to disable startup jobs, or to `thumb_derived_import,thumb_attached_import` to import without deleting. **Non-blocking** — readiness never waits on a job; entries run sequentially in the background. **Fail-fast** — an unknown job name or flag panics at boot, because a silently-dropped entry means a migration that never runs. A run interrupted by a restart resumes from its cursor on the next boot, so a long migration finishes across restarts. Safe to leave at the default: the jobs are idempotent, and once drained a run does nothing. See [Thumbnail Migration](./thumbnail-migration.md) for the upgrade runbook. |
+| `OXICLOUD_STARTUP_JOBS` | `thumb_derived_import?repair=true,thumb_attached_import?repair=true,transcode_import?repair=true` | Background jobs dispatched once at boot, comma-separated, each `name` or `name?flag=true` using the same syntax as `POST /api/admin/jobs/{name}/trigger`. Flags: `force`, `deep`, `repair`, `storage`. **The default migrates thumbnails out of the legacy `.thumbnails/` directory and deletes the originals**, so the migration completes without anyone triggering it from the admin panel; each sidecar is read back through the normal stack before it is unlinked, and every deletion is audited. An explicit value **replaces** the default; set it empty (`OXICLOUD_STARTUP_JOBS=`) to disable startup jobs, or to `thumb_derived_import,thumb_attached_import` to import without deleting. **Non-blocking** — readiness never waits on a job; entries run sequentially in the background. **Fail-fast** — an unknown job name or flag panics at boot, because a silently-dropped entry means a migration that never runs. A run interrupted by a restart resumes from its cursor on the next boot, so a long migration finishes across restarts. Safe to leave at the default: the jobs are idempotent, and once drained a run does nothing. See [Thumbnail Migration](./thumbnail-migration.md) for the upgrade runbook. |
 
 ## Database