refactor(tags): unify suggestion source — one canonical DB-tag dropdown, drop dead raw/alias machinery (#154)

Every tag suggestion is a canonical DB tag now (tagging-v2 #114: heads + CCIP
score EXISTING concept tags). The pre-heads apparatus for model-predicted tags
that didn't exist in the DB — creates_new_tag / raw_name / via_alias, the
/suggestions/alias endpoint + add_alias_and_accept, AliasPickerDialog, and the
store's aliasAccept/removeAlias — was dead and is removed.

The type-to-add dropdown was TWO row sources (server autocomplete + the image's
ML suggestions) merged with a dedup that dropped the %-bearing suggestion row
when the debounced server hit landed — the operator's "confidence % flickers
then vanishes". Now it's ONE list of DB-tag matches, each annotated with the
model's confidence (join by canonical_tag_id) when the tag was scored for this
image. No dedup, no flicker; picking a suggested tag still records acceptance
via TagPanel.findPending.

Single per-image fetch: score_image now reports above_threshold per row
(computed vs the head's own suggest cut, separate from the inclusion floor), so
the rail makes ONE min=0 request and derives the panel (above_threshold) and the
dropdown (all, text-filtered) client-side — the two /suggestions calls collapse
to one. Manual "Create 'X' as <kind>" (novel typed names) is unchanged; the
alias table + tag-side alias admin + auto-apply alias matching are untouched.

Tests: gate/serializer assertions updated (above_threshold; dropped dead-field
+ alias-endpoint checks); frontend spec seeds via the single load and covers the
byCategory/aboveByCategory split.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CgZP9v2otxVJymiYsnVuMy

tests/test_ml_suggestions.py

@@ -62,9 +62,8 @@ async def test_head_suggestion_surfaces_for_matching_image(db):
     s = general[0]
     assert s.canonical_tag_id == tag.id
     assert s.source == "head"
-    assert s.creates_new_tag is False
-    assert s.via_alias is False and s.raw_name is None
     assert s.score > 0.5
+    assert s.above_threshold is True   # ~0.73 clears the 0.5 suggest cut
 
 
 @pytest.mark.asyncio
@@ -109,7 +108,10 @@ async def test_threshold_override_surfaces_below_cut(db):
     svc = SuggestionService(db)
     assert svc and not (await svc.for_image(img.id)).by_category.get("general")
     flooded = await svc.for_image(img.id, threshold_override=0.0)
-    assert any(s.canonical_tag_id == tag.id for s in flooded.by_category["general"])
+    s = next(s for s in flooded.by_category["general"] if s.canonical_tag_id == tag.id)
+    # Included by the floor, but flagged below its own cut (0.5 < 0.6) — this is
+    # what lets the dropdown show it while the panel hides it.
+    assert s.above_threshold is False
 
 
 @pytest.mark.asyncio
@@ -281,6 +283,7 @@ async def test_ccip_character_surfaces_in_rail(db):
         if c.canonical_tag_id == raven.id
     )
     assert m.source == "ccip"
+    assert m.above_threshold is True   # CCIP only returns matches above its cut
 
 
 # --- #1206 Step 4: on-demand grounding for ALREADY-APPLIED tag chips ---------