feat: one image for every lane, with the model fetch gated on enabling (4296)

Milestone 422 step 6. Dockerfile.ml is gone; the main image carries torch,
torchvision, transformers, onnxruntime and opencv, and serves every lane.

WHY IT HAD TO MERGE: step 5 runs every lane in one process tree, so a second
image would mean the `ml` lane could never be enabled from the UI — there
would be no worker in that container to enable. The switch needs something to
switch.

THE MODEL NO LONGER DOWNLOADS AT BOOT. `entrypoint.sh`'s ml-worker role ran
download_models before celery started, so every boot of that role reached
HuggingFace for ~3.5GB — a startup dependency on a third party for a feature
the operator may never use. Rule 164 permits a runtime fetch only for
something "optional and clearly off", so the fetch is now a TASK, enqueued
the moment the lane is ENABLED.

Being a task is what makes it visible: it gets a TaskRun row, so the download
shows in Activity with a duration and a status, and a failure is something an
operator can see and retry rather than a container that quietly never became
useful. Idempotent, so re-enabling a provisioned lane costs one no-op.

Enqueued only when the lane actually came ON (`enabled is True`, not the
resolved value) so re-saving slots does not re-fetch, and only when the
consumer change landed — a task queued onto a queue nothing consumes would
sit pending with no explanation.

`fabledcurator-ml` KEEPS PUBLISHING, from the merged Dockerfile. The
operator's Swarm stack references that name and lives outside this repo;
dropping it would not break their deploy, it would freeze it silently at the
last publish — the exact failure class this milestone keeps finding. Retiring
the NAME is its own task, gated on that stack moving. Same two-phase shape
#406 used for pixiv.

THREE LIVE BREAKAGES from deleting the file, found by grepping for it rather
than assuming the build was the only consumer:

- `docker-compose.override.yml` built the ml service from it (contributor
  path would have failed at `docker compose build`).
- `tests/test_artifact_paths.py` pins the ml path set.
- `scripts/artifacts.sh` ML_PATHS named it. A path set naming a deleted file
  silently stops contributing to the derived revision — which the reuse check
  and the version string both read. That is #3202's recorded shape.

The `--with-ml` flag is gone from the generator and the healthcheck rather
than left defaulting to true. One image carries every lane now, so a flag
that can only be passed one way is a branch pretending to be a choice.

The advisory shipped in ecbd325 is what makes this honest to an adopter: the
lane says it is optional, names the model, and gives its download and
per-slot RAM before the switch is thrown.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

backend/app/tasks/ml.py

@@ -668,3 +668,31 @@ def scheduled_retract_auto_tags() -> str:
     with SessionLocal() as session:
         n_ccip = retract_auto_applied_ccip(session)
     return f"head={n_head} ccip={n_ccip}"
+
+
+@celery.task(name="backend.app.tasks.ml.ensure_models", bind=True)
+def ensure_models(self) -> dict:
+    """Fetch the models this lane needs, if they are not already present.
+
+    Milestone 422 step 6. This used to run in `entrypoint.sh` before celery
+    started, which made every boot of the ML role reach HuggingFace for
+    ~3.5GB — a startup dependency on a third party, for a feature the operator
+    may never use. Rule 164 permits a runtime fetch only for something
+    "optional and clearly off", so it moved here: enqueued the moment the lane
+    is ENABLED, never at boot.
+
+    Being a task rather than a startup step is what makes it visible: it gets
+    a TaskRun row like any other, so the download shows in Activity with a
+    duration and a status, and a failure is something the operator can see and
+    retry rather than a container that quietly never became useful.
+
+    Idempotent — `download_models` fetches only what is missing — so enabling
+    an already-provisioned lane costs one no-op task rather than a re-download.
+    That matters because the reconcile may enqueue it again.
+    """
+    from ..scripts.download_models import main as download
+
+    rc = download()
+    if rc != 0:
+        raise RuntimeError(f"model download failed with exit code {rc}")
+    return {"ok": True}