chore: retire ml-backfill-daily beat + the spent purge-legacy action (operator-approved)

- ml-backfill-daily: the CPU tag_and_embed backfill raced the GPU agent's
  daily embed backfill for the same NULL-embedding images at ~100x the cost
  (B1 audit verdict, milestone #124). The backfill TASK stays — the manual
  /api/ml/backfill button remains the deliberate CPU fallback pending B3.
- purge-legacy: one-time IR-migration cleanup, dry-run verified 0 targets on
  the live library before removal (A2 audit, milestone #123). Fully retired
  per rule 22: tile, store action, route, service fn, tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_api_admin.py

@@ -390,63 +390,6 @@ async def test_prune_unused_commit_deletes_and_returns_count(client, db):
     assert "byebye" in body["sample_names"]
 
 
-@pytest.mark.asyncio
-async def test_purge_legacy_dry_run_counts_by_kind_and_prefix(client, db):
-    db.add_all([
-        Tag(name="BlenderKnight:Hannah_BJ_Loops", kind=TagKind.archive),
-        Tag(name="BlenderKnight:May Animation", kind=TagKind.post),
-        Tag(name="SomeArtist", kind=TagKind.artist),
-        # IR `source` kind fell back to general during migration —
-        # caught by the source:* name prefix, not by kind.
-        Tag(name="source:patreon", kind=TagKind.general),
-        Tag(name="blonde hair", kind=TagKind.general),  # must survive
-    ])
-    await db.commit()
-
-    resp = await client.post(
-        "/api/admin/tags/purge-legacy", json={"dry_run": True},
-    )
-    body = await resp.get_json()
-    assert body["count"] == 4
-    assert body["by_kind"] == {"archive": 1, "post": 1, "artist": 1}
-    assert body["by_prefix"] == {"source:*": 1}
-    assert "blonde hair" not in body["sample_names"]
-    assert "source:patreon" in body["sample_names"]
-
-
-@pytest.mark.asyncio
-async def test_purge_legacy_commit_deletes_only_legacy(client, db):
-    from sqlalchemy import func, select
-
-    db.add_all([
-        Tag(name="arch1", kind=TagKind.archive),
-        Tag(name="source:fanbox", kind=TagKind.general),
-        Tag(name="keepme", kind=TagKind.general),
-        Tag(name="char1", kind=TagKind.character),
-    ])
-    await db.commit()
-
-    resp = await client.post(
-        "/api/admin/tags/purge-legacy", json={"dry_run": False},
-    )
-    body = await resp.get_json()
-    assert body["deleted"] == 2  # arch1 + source:fanbox
-
-    # The plain general + character tags survive.
-    keep = (await db.execute(
-        select(Tag.name).where(Tag.name.in_(["keepme", "char1"]))
-    )).scalars().all()
-    assert set(keep) == {"keepme", "char1"}
-    # No archive/post/artist or source:* left.
-    gone = (await db.execute(
-        select(func.count()).select_from(Tag).where(
-            (Tag.kind.in_([TagKind.archive, TagKind.post, TagKind.artist]))
-            | (Tag.name.like("source:%"))
-        )
-    )).scalar_one()
-    assert gone == 0
-
-
 # --- Tier-A: POST /posts/prune-bare + /posts/reconcile-duplicates ---
 # These two routes share _run_dry_run_op with the tag prunes (DRY pass,
 # task #753); cover their apply path + reconcile's source_id passthrough so