feat(heads): surface system-tag suggestions at a flat 0.65 confidence floor

System tags (wip/banner/editor) already get heads (kind=general) and aren't
filtered from suggestions, but they surfaced only at each head's precision-tuned
suggest_threshold — high enough to hide the borderline/false-positive guesses the
operator wants to SEE and REJECT (hard-negative mining: 'negatively reinforce
what isn't a system tag'). score_image now uses a flat _SYSTEM_TAG_SUGGEST_FLOOR
(0.65, operator-set) for system-tag heads instead of their auto threshold;
content-tag heads keep their own, and the typed-dropdown threshold_override still
overrides everything. _current_heads carries Tag.is_system into the head meta to
drive it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_ml_suggestions.py

@@ -111,6 +111,49 @@ async def test_threshold_override_surfaces_below_cut(db):
     assert any(s.canonical_tag_id == tag.id for s in flooded.by_category["general"])
 
 
+@pytest.mark.asyncio
+async def test_system_tag_surfaces_at_flat_floor_despite_high_threshold(db):
+    # A system tag's head uses the flat _SYSTEM_TAG_SUGGEST_FLOOR (0.65), not its
+    # precision-tuned suggest_threshold — so a matching image (~0.73) surfaces it
+    # for rejection even though its 0.9 auto threshold would hide it.
+    from backend.app.services.ml.heads import _SYSTEM_TAG_SUGGEST_FLOOR
+    assert _SYSTEM_TAG_SUGGEST_FLOOR == 0.65
+    tag = await TagService(db).find_or_create("wip sketchy", TagKind.general)
+    tag.is_system = True
+    img = await _img(db, "1" * 64, _emb(0))            # matches head → ~0.73
+    await _head(db, tag.id, slot=0, suggest_threshold=0.9)
+    await db.commit()
+    sl = await SuggestionService(db).for_image(img.id)
+    assert any(
+        s.canonical_tag_id == tag.id for s in sl.by_category.get("general", [])
+    )
+
+
+@pytest.mark.asyncio
+async def test_system_tag_below_floor_stays_hidden(db):
+    # Below the 0.65 floor (~0.5 on an orthogonal image) the system tag is not
+    # surfaced — the floor still gates, it isn't "always show".
+    tag = await TagService(db).find_or_create("wip faint", TagKind.general)
+    tag.is_system = True
+    img = await _img(db, "2" * 64, _emb(1))            # orthogonal → 0.5
+    await _head(db, tag.id, slot=0, suggest_threshold=0.9)
+    await db.commit()
+    sl = await SuggestionService(db).for_image(img.id)
+    assert not sl.by_category.get("general")
+
+
+@pytest.mark.asyncio
+async def test_content_tag_ignores_system_floor(db):
+    # A NON-system head with the same high threshold does NOT get the floor: the
+    # ~0.73 match stays hidden below its 0.9 auto threshold (floor is system-only).
+    tag = await TagService(db).find_or_create("glasses hi", TagKind.general)
+    img = await _img(db, "3" * 64, _emb(0))            # matches head → ~0.73
+    await _head(db, tag.id, slot=0, suggest_threshold=0.9)
+    await db.commit()
+    sl = await SuggestionService(db).for_image(img.id)
+    assert not sl.by_category.get("general")
+
+
 @pytest.mark.asyncio
 async def test_concept_region_surfaces_via_max_over_bag(db):
     # Max-over-bag: the whole-image vector is orthogonal to the head (scores the