feat(gallery,tags): clear active filters

Two gaps where a filter couldn't be removed:

- Gallery: a tag_id filter (from clicking a tag) had no indicator or clear
  control — only post_id did (PostInfoHeader). Add an "Tag: <name> ✕" chip
  that clears the filter by dropping tag_id from the URL. New lightweight
  GET /api/tags/<id> resolves the name; the store fetches it on filter set.
- Tags view: the kind chip-group used mandatory="false" — a STRING ("false"
  is truthy in JS), which made the group mandatory so the active kind chip
  couldn't be deselected. Fixed to :mandatory="false" so the filter clears.

Tests: GET /tags/<id> shape + 404; gallery store resolves filterTagName.

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

backend/app/api/tags.py

@@ -194,6 +194,24 @@ async def remove_tag_from_image(image_id: int, tag_id: int):
     return "", 204
 
 
+@tags_bp.route("/tags/<int:tag_id>", methods=["GET"])
+async def get_tag(tag_id: int):
+    """Resolve a single tag (used by the gallery to label its active
+    tag-filter chip)."""
+    async with get_session() as session:
+        tag = await session.get(Tag, tag_id)
+        if tag is None:
+            return jsonify({"error": "tag not found"}), 404
+        return jsonify(
+            {
+                "id": tag.id,
+                "name": tag.name,
+                "kind": tag.kind.value,
+                "fandom_id": tag.fandom_id,
+            }
+        )
+
+
 @tags_bp.route("/tags/<int:tag_id>", methods=["PATCH"])
 async def update_tag(tag_id: int):
     """Rename and/or re-fandom a tag. Body may carry `name` and/or

frontend/src/stores/gallery.js

@@ -22,6 +22,9 @@ export const useGalleryStore = defineStore('gallery', () => {
   const loading = ref(false)
   const error = ref(null)
   const filter = ref({ tag_id: null, post_id: null })
+  // Display name for the active tag filter, so the gallery can label a
+  // "Tag: X ✕" chip instead of leaving the filter invisible/unclearable.
+  const filterTagName = ref(null)
 
   const timelineBuckets = ref([])
   const timelineLoading = ref(false)
@@ -94,10 +97,20 @@ export const useGalleryStore = defineStore('gallery', () => {
   function setTagFilter(tagId) {
     filter.value.tag_id = tagId
     filter.value.post_id = null
+    filterTagName.value = null
+    if (tagId) _resolveTagName(tagId)  // fire-and-forget; chip label only
     loadInitial()
     loadTimeline()
   }
 
+  async function _resolveTagName(tagId) {
+    try {
+      filterTagName.value = (await api.get(`/api/tags/${tagId}`)).name
+    } catch {
+      // Leave null — the chip falls back to "#<id>".
+    }
+  }
+
   function setPostFilter(postId) {
     filter.value.post_id = postId
     filter.value.tag_id = null
@@ -110,7 +123,7 @@ export const useGalleryStore = defineStore('gallery', () => {
 
   return {
     images, dateGroups, hasMore, isEmpty, loading, error,
-    filter, timelineBuckets, timelineLoading,
+    filter, filterTagName, timelineBuckets, timelineLoading,
     loadInitial, loadMore, loadTimeline, jumpTo, setTagFilter, setPostFilter
   }
 })

frontend/src/views/GalleryView.vue

@@ -12,6 +12,13 @@
     <div class="fc-gallery-layout">
       <div class="fc-gallery-layout__main">
         <PostInfoHeader />
+        <div v-if="store.filter.tag_id != null" class="fc-gallery-activefilter">
+          <v-chip
+            color="accent" variant="tonal" closable
+            prepend-icon="mdi-tag"
+            @click:close="clearTagFilter"
+          >Tag: {{ store.filterTagName || `#${store.filter.tag_id}` }}</v-chip>
+        </div>
         <EmptyState v-if="store.isEmpty" />
         <GalleryGrid v-else @open="openImage" />
       </div>
@@ -24,7 +31,7 @@
 
 <script setup>
 import { onMounted, watch } from 'vue'
-import { useRoute } from 'vue-router'
+import { useRoute, useRouter } from 'vue-router'
 import { useGalleryStore } from '../stores/gallery.js'
 import { useModalStore } from '../stores/modal.js'
 import GalleryGrid from '../components/gallery/GalleryGrid.vue'
@@ -38,6 +45,7 @@ const store = useGalleryStore()
 const modal = useModalStore()
 const sel = useGallerySelectionStore()
 const route = useRoute()
+const router = useRouter()
 
 onMounted(async () => {
   const postId = parseInt(route.query.post_id, 10)
@@ -63,6 +71,14 @@ watch(() => route.query.post_id, (q) => {
 function openImage(id) {
   modal.open(id)
 }
+
+// Clear the active tag filter by dropping it from the URL; the route
+// watcher above resets the store filter and reloads.
+function clearTagFilter() {
+  const q = { ...route.query }
+  delete q.tag_id
+  router.push({ query: q })
+}
 </script>
 
 <style scoped>
@@ -72,6 +88,7 @@ function openImage(id) {
   align-items: flex-start;
 }
 .fc-gallery-layout__main { flex: 1; min-width: 0; }
+.fc-gallery-activefilter { margin-bottom: 12px; }
 .fc-gallery-layout__sidebar { flex-shrink: 0; }
 
 @media (max-width: 900px) {

frontend/src/views/TagsView.vue

@@ -8,7 +8,7 @@
         placeholder="Search tags" clearable style="max-width: 320px;"
       />
       <v-chip-group
-        v-model="kind" selected-class="text-accent" mandatory="false"
+        v-model="kind" selected-class="text-accent" :mandatory="false"
       >
         <v-chip v-for="k in KINDS" :key="k" :value="k" filter size="small">
           {{ k }}

frontend/test/gallery.spec.js

@@ -50,6 +50,19 @@ describe('gallery store: tag/post filter exclusivity', () => {
     expect(scrollCall).not.toContain('tag_id=')
   })
 
+  it('setTagFilter resolves the tag name for the filter chip', async () => {
+    const s = useGalleryStore()
+    stubFetch((url) => {
+      if (url.includes('/api/tags/7')) {
+        return { status: 200, body: { id: 7, name: 'Asuka', kind: 'character' } }
+      }
+      return { status: 200, body: EMPTY }  // scroll + timeline
+    })
+    s.setTagFilter(7)
+    expect(s.filter.tag_id).toBe(7)
+    await vi.waitFor(() => expect(s.filterTagName).toBe('Asuka'))
+  })
+
   it('loadInitial issues exactly one scroll request at the initial limit', async () => {
     const s = useGalleryStore()
     const urls = []

tests/test_api_tag_merge.py

@@ -11,6 +11,19 @@ async def _mk(client, name, kind, fandom_id=None):
     return (await r.get_json())["id"]
 
 
+@pytest.mark.asyncio
+async def test_get_tag_returns_shape_and_404(client):
+    tid = await _mk(client, "Lookup", "general")
+    resp = await client.get(f"/api/tags/{tid}")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["id"] == tid
+    assert body["name"] == "Lookup"
+    assert body["kind"] == "general"
+    resp = await client.get("/api/tags/99999999")
+    assert resp.status_code == 404
+
+
 @pytest.mark.asyncio
 async def test_patch_sets_and_clears_character_fandom(client):
     fandom = await _mk(client, "Bleach", "fandom")