test(transcode): pin transcode_import's contract and empty-tree run

Covers the job's surface: that it is registered, declares the metadata
the admin panel switches on (`mutates: always`, recoverable, a repair
description), and that a run against a drained tree completes cleanly
with zeroed counters.

It deliberately does NOT cover the re-keying, which is the part that
matters most. That needs `.transcoded/webp/` entries on disk before the
run, and nothing reachable over HTTP can create them — since the write
path moved to the derived tier, only hash-less callers still write
there, and hurl cannot place files in the server's storage directory.
The migration is validated by a snapshot restore instead, the way the
thumbnail one was; the file says so rather than implying coverage it
does not have.

The empty-tree assertions still earn their place. A drained tree is
what every run after the first sees, so it is the overwhelming
majority of this job's lifetime, and "does nothing, quietly" is a real
property: the thumbnail teardown warned `could not be removed / No
such file or directory` on every boot after its migration finished —
warning about success forever — and that was caught by eye, not by a
test. Counters are asserted as exact zeros, since finding work in a
directory the API cannot populate is the shape a re-keying bug would
take.

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

tests/api/run.sh

@@ -170,6 +170,7 @@ hurl --variables-file "$API_DIR/test.env" --file-root "$REPO_ROOT/tests" --test
   "$API_DIR/thumbnail_etag_content_keyed.hurl" \
   "$API_DIR/attached_thumbnail_copy.hurl" \
   "$API_DIR/transcode_cache.hurl" \
+  "$API_DIR/transcode_import.hurl" \
   "$API_DIR/dedup_admin_gate.hurl" \
   "$API_DIR/admin_jobs.hurl" \
   "$API_DIR/recoverable_jobs.hurl" \