feat(tags): legacy-tag purge also catches source:* general tags

Operator-confirmed 2026-05-28: the BlenderKnight:* tags are `archive`
kind (caught by the kind purge), but the `source:patreon`-style tags
are IR's old `source` kind that fell back to `general` during migration
(FC's enum has no `source` kind) — so they can't be matched by kind.

Broadened the purge to a two-rule match and renamed it for accuracy
(all dev-only, unreleased):
- cleanup_service.purge_tags_by_kind → purge_legacy_tags. Predicate is
  now `kind IN (archive, post, artist) OR name LIKE 'source:%'`
  (LEGACY_NAME_PREFIXES). Preview classifies each row into by_kind OR
  by_prefix (source:* counts once under the prefix bucket regardless of
  its general kind).
- endpoint /tags/purge-retired-kinds → /tags/purge-legacy. dry-run
  returns by_kind + by_prefix + count + sample.
- store purgeRetiredKindTags → purgeLegacyTags.
- Tag Maintenance card copy + breakdown updated to show both buckets;
  button reads "Preview/Delete legacy tags".

Tests updated + extended: dry-run reports by_kind {archive,post,artist}
AND by_prefix {source:*}, plain general/character tags survive; commit
deletes both the kind-matched and source:*-matched rows and leaves the
rest.

frontend/src/components/settings/TagMaintenanceCard.vue

@@ -48,12 +48,14 @@
       <v-divider class="my-5" />
 
       <p class="fc-muted text-body-2 mb-4">
-        Purge tags with retired/system kinds FC no longer uses
-        (<code>archive</code>, <code>post</code>, <code>artist</code>) —
-        IR-migration leftovers like
-        <code>BlenderKnight:Hannah_BJ_Loops</code>. Provenance and
-        artists are their own systems now, so these tag rows are pure
-        noise. Removes them from every image.
+        Purge legacy IR-migration tags FC no longer uses: retired/system
+        kinds (<code>archive</code>, <code>post</code>,
+        <code>artist</code> — e.g.
+        <code>BlenderKnight:Hannah_BJ_Loops</code>) plus
+        <code>source:*</code> tags (ImageRepo's old <code>source</code>
+        kind, which migrated to <code>general</code>). Provenance and
+        artists are their own systems now, so these are pure noise.
+        Removes them from every image.
       </p>
 
       <v-btn
@@ -62,14 +64,17 @@
         :loading="loadingKindPreview"
         class="mb-3"
         @click="onKindPreview"
-      >Preview retired-kind tags</v-btn>
+      >Preview legacy tags</v-btn>
 
       <div v-if="kindPreview">
         <p class="text-body-2 mb-2">
-          <strong>{{ kindPreview.count }}</strong> tag(s) of retired kinds.
+          <strong>{{ kindPreview.count }}</strong> legacy tag(s).
           <span v-for="(n, k) in kindPreview.by_kind" :key="k" class="fc-muted">
             {{ k }}: {{ n }}&nbsp;&nbsp;
           </span>
+          <span v-for="(n, p) in kindPreview.by_prefix" :key="p" class="fc-muted">
+            {{ p }}: {{ n }}&nbsp;&nbsp;
+          </span>
         </p>
         <div v-if="kindPreview.sample_names?.length" class="fc-name-grid mb-3">
           <span v-for="n in kindPreview.sample_names" :key="n" class="fc-name">
@@ -82,7 +87,7 @@
           :disabled="!kindPreview.count"
           :loading="kindCommitting"
           @click="onKindCommit"
-        >Delete {{ kindPreview.count }} retired-kind tag(s)</v-btn>
+        >Delete {{ kindPreview.count }} legacy tag(s)</v-btn>
       </div>
     </v-card-text>
   </v-card>
@@ -123,7 +128,7 @@ async function onCommit() {
 async function onKindPreview() {
   loadingKindPreview.value = true
   try {
-    kindPreview.value = await store.purgeRetiredKindTags({ dryRun: true })
+    kindPreview.value = await store.purgeLegacyTags({ dryRun: true })
   } finally {
     loadingKindPreview.value = false
   }
@@ -132,8 +137,8 @@ async function onKindPreview() {
 async function onKindCommit() {
   kindCommitting.value = true
   try {
-    await store.purgeRetiredKindTags({ dryRun: false })
-    kindPreview.value = { count: 0, by_kind: {}, sample_names: [] }
+    await store.purgeLegacyTags({ dryRun: false })
+    kindPreview.value = { count: 0, by_kind: {}, by_prefix: {}, sample_names: [] }
   } finally {
     kindCommitting.value = false
   }