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

README.md

@@ -265,7 +265,7 @@ Five deployable pieces, built by `.forgejo/workflows/build.yml`:
 | Piece | Built from | Image | Role |
 | --- | --- | --- | --- |
 | **Web / workers** | `Dockerfile` | `fabledcurator` | Quart API + the built Vue SPA in one image. `entrypoint.sh` picks the role: `web`, `worker`, `scheduler`. The `maintenance-long` service is a second `worker` pinned to the long-running maintenance queue. |
-| **ML worker** | `Dockerfile.ml` | `fabledcurator-ml` | Same app, plus `requirements-ml.txt` — tagging and embedding models that run in-container. |
+| **ML worker** | `Dockerfile` | `fabledcurator-ml` | The same image as the web service since milestone 422 — one image serves every lane. Published under this name too, for stacks that still reference it. |
 | **GPU agent** | `agent/Dockerfile` | `fabledcurator-agent` | Optional desktop-GPU worker (`agent/`). Leases jobs over **HTTP only** — never touches the database or Redis. See `agent/README.md`. |
 | **Firefox extension** | `extension/` | signed XPI | MV3 extension: pushes platform session cookies into FC and adds a creator as a Source in one click. AMO-signed on both `dev` and `main` (one signature per extension change, shared by the two channels), bundled into that channel's web image and served from Settings → Maintenance. See `extension/README.md`. |
 | **Data** | — | `pgvector/pgvector:pg16`, `redis:7-alpine` | Postgres with pgvector for embeddings; Redis as the Celery broker. |