feat(ml): drop image_record.tagger_predictions — image_prediction is sole store (#768 step 3)

Read cutover verified in prod (suggestions + allowlist read image_prediction;
backfill complete at 908k rows / 51k images). Removes the old JSON column and
everything that fed it:

- ImageRecord.tagger_predictions column removed; migration 0046 DROPs it.
  tagger_model_version kept as the "tagged / current?" signal the backfill
  sweep reads (needs-tagging check switched to tagger_model_version IS NULL).
- tag_and_embed no longer dual-writes the JSON — image_prediction is the only
  write path.
- importer re-import reset drops the JSON line (image_prediction rows are
  already deleted on re-import).
- Retired the one-time #768 backfill task + the #764 prune task, their admin
  endpoints, and their Maintenance cards (Backfill/PrunePredictionsCard).
- Tests seed/assert via image_prediction; stale column refs removed.

Disk reclaim is NOT automatic: DROP COLUMN is a catalog change. Run
`VACUUM FULL image_record` off-hours afterward to return the ~100 GB to the OS
so DB backups go small (#739). image_prediction (~90 MB) stays in pg_dump — it's
the source of truth now.

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

backend/app/api/admin.py

@@ -251,7 +251,7 @@ async def tags_reset_content():
     """Tier-A: delete ALL general + character tags (the Camie-suggestable
     content vocabulary) so the operator can re-tag from scratch via
     auto-suggest. fandom + series tags + series_page ordering are preserved,
-    and image tagger_predictions are untouched so suggestions repopulate.
+    and image_prediction rows are untouched so suggestions repopulate.
     dry-run preview returns per-kind counts + applications + a sample so the
     UI shows exactly what'll go before the operator confirms (dry_run=false).
     Irreversible except via DB backup restore."""
@@ -348,28 +348,3 @@ async def trigger_reextract_archives():
 
     async_result = reextract_archive_attachments_task.delay()
     return jsonify({"task_id": async_result.id, "status": "queued"}), 202
-
-
-@admin_bp.route("/maintenance/prune-predictions", methods=["POST"])
-async def trigger_prune_predictions():
-    """Operator-triggered #764 backfill: drop stored tagger predictions below
-    the current ml_settings.tagger_store_floor and clamp allowlist thresholds
-    up to it. Shrinks image_record's TOAST (~100 GB of sub-0.70 scores).
-    Idempotent + self-resuming; runs on the maintenance_long lane."""
-    from ..tasks.admin import prune_low_confidence_predictions_task
-
-    async_result = prune_low_confidence_predictions_task.delay()
-    return jsonify({"task_id": async_result.id, "status": "queued"}), 202
-
-
-@admin_bp.route("/maintenance/backfill-predictions", methods=["POST"])
-async def trigger_backfill_predictions():
-    """Operator-triggered #768 backfill: copy stored tagger predictions from the
-    image_record.tagger_predictions JSON into the normalized image_prediction
-    table. Batched + resumable + idempotent; runs on the maintenance_long lane.
-    Run this once after deploying migration 0045 (which creates the empty table)
-    to populate predictions for the existing library."""
-    from ..tasks.admin import backfill_image_predictions_task
-
-    async_result = backfill_image_predictions_task.delay()
-    return jsonify({"task_id": async_result.id, "status": "queued"}), 202