fix(aliases): store modal alias under raw model key + make aliases visible/manageable

The headline bug: aliases created from the modal NEVER resolved. Create
sent the normalized display name ('Sword', 'Uchiha Sasuke') while
resolution keys on the raw booru model key ('sword', 'uchiha_sasuke',
case-sensitive) — so the mapping was stored under a key nothing looks up,
and the prediction kept reappearing unaliased. The raw key wasn't even in
the /suggestions response, so the modal couldn't send it.

- Suggestion now carries raw_name (the model key an alias must use) and
  via_alias (surfaced via an operator alias); both serialized by the API.
- Modal alias-create sends raw_name, not display_name (the fix). Aliased
  suggestions show an 'alias' badge and a 'Remove alias' action; 'Treat as
  alias for…' is hidden for centroid hits (no model key) and already-aliased
  rows.
- Tag-side management: TagCard ⋮ → 'Aliases…' opens a dialog listing the
  model keys that fold into a tag, with remove (GET /api/tags/<id>/aliases +
  AliasService.list_for_tag). Creation stays in the modal suggestion flow.

Tests: full API round-trip locking the raw-key contract (raw_name exposed →
alias authored with it → resolves + via_alias on a later image);
list_for_tag (service + API); via_alias/raw_name on the existing service
suggestion tests. No migration.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/modal/SuggestionItem.vue

@@ -8,6 +8,8 @@
       {{ suggestion.display_name }}
       <span v-if="suggestion.creates_new_tag" class="fc-suggestion__new"
         title="No matching tag yet — accepting creates it">+ new</span>
+      <span v-else-if="suggestion.via_alias" class="fc-suggestion__alias"
+        :title="`Mapped from the tagger's “${suggestion.raw_name}” via an alias`">alias</span>
     </span>
     <span class="fc-suggestion__score">{{ scorePct }}</span>
     <v-btn
@@ -25,9 +27,21 @@
       class="fc-suggestion__menu" size="small" variant="outlined"
       :label="`More actions for ${suggestion.display_name}`"
     >
-      <v-list-item @click="$emit('alias', suggestion)">
+      <!-- Alias is a tagger-prediction remap, so only offer it for tagger
+           suggestions with a raw model key that aren't already aliased.
+           Centroid hits (raw_name null) have nothing to alias. -->
+      <v-list-item
+        v-if="suggestion.raw_name && !suggestion.via_alias"
+        @click="$emit('alias', suggestion)"
+      >
         <v-list-item-title>Treat as alias for…</v-list-item-title>
       </v-list-item>
+      <v-list-item
+        v-if="suggestion.via_alias"
+        @click="$emit('remove-alias', suggestion)"
+      >
+        <v-list-item-title>Remove alias</v-list-item-title>
+      </v-list-item>
       <v-list-item @click="$emit('dismiss', suggestion)">
         <v-list-item-title>Dismiss for this image</v-list-item-title>
       </v-list-item>
@@ -40,7 +54,7 @@ import { computed } from 'vue'
 import KebabMenu from '../common/KebabMenu.vue'
 
 const props = defineProps({ suggestion: { type: Object, required: true } })
-defineEmits(['accept', 'alias', 'dismiss'])
+defineEmits(['accept', 'alias', 'remove-alias', 'dismiss'])
 
 const scorePct = computed(() => `${Math.round(props.suggestion.score * 100)}%`)
 </script>
@@ -74,6 +88,16 @@ const scorePct = computed(() => `${Math.round(props.suggestion.score * 100)}%`)
   margin-left: 6px;
   text-transform: uppercase; letter-spacing: 0.04em;
 }
+.fc-suggestion__alias {
+  display: inline-block;
+  font-size: 10px; font-weight: 600;
+  color: rgb(var(--v-theme-on-surface-variant));
+  background: rgb(var(--v-theme-surface-light));
+  border: 1px solid rgb(var(--v-theme-surface-light));
+  padding: 1px 6px; border-radius: 999px;
+  margin-left: 6px;
+  text-transform: uppercase; letter-spacing: 0.04em;
+}
 .fc-suggestion__score {
   flex: 0 0 auto; min-width: 38px; text-align: right;
   font-size: 11px;