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

frontend/src/components/modal/SuggestionsCategoryGroup.vue

@@ -26,8 +26,6 @@
         v-for="(s, i) in items" :key="`${s.display_name}-${i}`"
         :suggestion="s"
         @accept="$emit('accept', $event)"
-        @alias="$emit('alias', $event)"
-        @remove-alias="$emit('remove-alias', $event)"
         @dismiss="$emit('dismiss', $event)"
         @undismiss="$emit('undismiss', $event)"
       />
@@ -45,7 +43,7 @@ const props = defineProps({
   collapsible: { type: Boolean, default: false },
   defaultOpen: { type: Boolean, default: true }
 })
-defineEmits(['accept', 'alias', 'remove-alias', 'dismiss', 'undismiss', 'reject-all'])
+defineEmits(['accept', 'dismiss', 'undismiss', 'reject-all'])
 
 // Still-unhandled suggestions (not yet rejected) — how many "Reject rest" clears.
 const rejectableCount = computed(() => props.items.filter((s) => !s.rejected).length)