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_api_suggestions.py

@@ -52,6 +52,7 @@ async def test_get_suggestions(client, db):
     general = body["by_category"].get("general", [])
     s2 = next(x for x in general if x["canonical_tag_id"] == tag.id)
     assert s2["source"] == "head"
+    assert s2["above_threshold"] is True   # ~0.73 clears the 0.5 suggest cut
 
 
 @pytest.mark.asyncio
@@ -114,12 +115,3 @@ async def test_undismiss_reverses_rejection(client, db):
         f"/api/images/{img.id}/suggestions/undismiss", json={"tag_id": tag.id}
     )
     assert resp2.status_code == 204
-
-
-@pytest.mark.asyncio
-async def test_alias_requires_fields(client, db):
-    img = await _img(db)
-    resp = await client.post(
-        f"/api/images/{img.id}/suggestions/alias", json={"alias_string": "x"}
-    )
-    assert resp.status_code == 400