feat(ui): hover an applied tag chip → highlight its grounding crop (#133 step 4)

Applied tags aren't scored live, so compute the grounding on demand: run the
tag's head over the image's max-over-bag (whole-image + concept crops), argmax
→ the region that best explains the tag on this image, mirroring what
score_image records for live suggestions.

- heads.py: extract _image_bag (now shared by score_image) + ground_applied_tag.
  Returns (grounding, has_head): has_head False = no head to localize with →
  no overlay; grounding None = the whole-image vector won → whole-image frame.
- tags.py: GET /api/images/<id>/tags/<id>/grounding → {grounding, has_head}.
- TagChip/TagPanel: applied chips inject fcSuggestionHover and fetch grounding
  on hover (cached per image+tag, race-guarded), reusing Step 3's overlay in
  both the modal and Explore. No new frontend overlay code.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/tags.py

@@ -11,6 +11,7 @@ from ..models.tag import image_tag
 from ..models.tag_suggestion_rejection import TagSuggestionRejection
 from ..services.bulk_tag_service import BulkTagService
 from ..services.ml.aliases import AliasService
+from ..services.ml.heads import ground_applied_tag
 from ..services.series_match_service import SeriesMatchService
 from ..services.series_service import SeriesError, SeriesService
 from ..services.tag_directory_service import TagDirectoryService
@@ -310,6 +311,21 @@ async def confirm_tag_on_image(image_id: int, tag_id: int):
     return "", 204
 
 
+@tags_bp.route(
+    "/images/<int:image_id>/tags/<int:tag_id>/grounding", methods=["GET"]
+)
+async def tag_grounding(image_id: int, tag_id: int):
+    """Which crop region best explains an ALREADY-APPLIED tag on this image
+    (#1206 Step 4). Powers the hover→overlay highlight on applied tag chips,
+    mirroring the suggestion rail's live grounding. Computed on demand (applied
+    tags aren't scored live). → {grounding: {bbox,kind,detector}|null,
+    has_head: bool}; has_head False means the tag has no head to localize with,
+    so the chip shows no overlay."""
+    async with get_session() as session:
+        grounding, has_head = await ground_applied_tag(session, image_id, tag_id)
+    return jsonify({"grounding": grounding, "has_head": has_head})
+
+
 @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