feat(ml): tag-eval backend — head-vs-centroid learning-curve eval (persisted)

Slice 1 of milestone #114 (tagging v2). Proves the frozen-embedding + trained-
head spine on the operator's own data, reusing the SigLIP embeddings already
stored on image_record — no re-embedding, no GPU.

Per concept: train a logistic-regression HEAD (positives + negatives = explicit
rejections + sampled unlabeled) vs the old single-CENTROID baseline; report
cross-validated precision/recall/AP for both, a LEARNING CURVE (AP/F1 as tagged
positives grow 10→30→100→300), and example image ids (head-would-suggest /
head-doubts-positive) to eyeball.

Persisted so the report SURVIVES navigation (operator-flagged): the run + full
report live in a new tag_eval_run row (mirrors library_audit_run); the admin
card will rehydrate from GET on mount, not transient state.

- models.TagEvalRun + migration 0056; runs on the ml queue (only worker with
  numpy/sklearn) — numpy/sklearn lazy-imported so the API can still enqueue.
- services/ml/tag_eval (compute + start helper, one-running guard), tasks.ml
  .tag_eval_run, api/tag-eval (POST create, GET history light / detail w/ report).
- recover_stalled_tag_eval_runs sweep + retention (keep last 20) + 5-min beat
  (rule 89). scikit-learn added to requirements-ml.
- tests: param normalization + the rehydrate read-path + create/conflict.

Frontend admin card (trigger + render persisted report) follows next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/__init__.py

@@ -36,6 +36,7 @@ def all_blueprints() -> list[Blueprint]:
     from .suggestions import suggestions_bp
     from .system_activity import system_activity_bp
     from .system_backup import system_backup_bp
+    from .tag_eval import tag_eval_bp
     from .tags import tags_bp
     from .thumbnails import thumbnails_bp
     return [
@@ -56,6 +57,7 @@ def all_blueprints() -> list[Blueprint]:
         suggestions_bp,
         allowlist_bp,
         aliases_bp,
+        tag_eval_bp,
         ml_admin_bp,
         thumbnails_bp,
         sources_bp,

backend/app/api/tag_eval.py

@@ -0,0 +1,70 @@
+"""Tag-eval API (#1130): trigger + revisit the head-vs-centroid eval.
+
+The run + full report live in the tag_eval_run row, so the admin card rehydrates
+from GET (history / detail) on mount — the report survives navigation rather than
+living in transient frontend state.
+"""
+
+from quart import Blueprint, jsonify, request
+from sqlalchemy import select
+
+from ..extensions import get_session
+from ..models import TagEvalRun
+from ..services.ml.tag_eval import EvalAlreadyRunning, start_tag_eval_run
+
+tag_eval_bp = Blueprint("tag_eval", __name__, url_prefix="/api/tag-eval")
+
+
+def _serialize(run: TagEvalRun, *, include_report: bool) -> dict:
+    out = {
+        "id": run.id,
+        "params": run.params,
+        "status": run.status,
+        "started_at": run.started_at.isoformat() if run.started_at else None,
+        "finished_at": run.finished_at.isoformat() if run.finished_at else None,
+        "error": run.error,
+    }
+    if include_report:
+        out["report"] = run.report
+    return out
+
+
+@tag_eval_bp.route("", methods=["POST"])
+async def create():
+    body = await request.get_json(silent=True) or {}
+    params = body.get("params") or body or {}
+    async with get_session() as session:
+        try:
+            run_id = await session.run_sync(
+                lambda s: start_tag_eval_run(s, params)
+            )
+        except EvalAlreadyRunning as running:
+            return jsonify({
+                "error": "eval_already_running",
+                "running_id": int(running.args[0]),
+            }), 409
+        await session.commit()
+    return jsonify({"run_id": run_id, "status": "running"}), 202
+
+
+@tag_eval_bp.route("", methods=["GET"])
+async def history():
+    try:
+        limit = min(int(request.args.get("limit", "20")), 100)
+    except ValueError:
+        return jsonify({"error": "invalid_limit"}), 400
+    async with get_session() as session:
+        rows = (await session.execute(
+            select(TagEvalRun).order_by(TagEvalRun.id.desc()).limit(limit)
+        )).scalars().all()
+        # List is light — no full report (the detail endpoint carries it).
+        return jsonify({"runs": [_serialize(r, include_report=False) for r in rows]})
+
+
+@tag_eval_bp.route("/<int:run_id>", methods=["GET"])
+async def detail(run_id: int):
+    async with get_session() as session:
+        run = await session.get(TagEvalRun, run_id)
+        if run is None:
+            return jsonify({"error": "not_found"}), 404
+        return jsonify(_serialize(run, include_report=True))