feat(ml): argmax grounding in score_image → suggestions carry the winning crop (#133 step 1)

score_image now keeps the ARGMAX beside the max-over-bag: which bag row won each
head. The region query also selects bbox/kind/detector_version, a parallel
bag_meta maps each row → its region (None for the whole-image vector), and every
hit gains grounding {bbox,kind,detector} (null when the global vector won). Threaded
through SuggestionService (new Suggestion.grounding field) → /api/.../suggestions
payload. This is the data the #1206 hover-overlay draws. CCIP-only hits ground null
for now (figure grounding = step 2). Tests: winning crop grounds the tag with its
bbox+kind; whole-image win → grounding None.

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

backend/app/api/suggestions.py

@@ -46,6 +46,10 @@ async def get_suggestions(image_id: int):
                         # (not dropped) so the rail can show it rejected + offer
                         # one-click un-reject.
                         "rejected": s.rejected,
+                        # the crop region that produced this tag (#1206) —
+                        # {bbox,kind,detector} or null (whole-image won). Drives
+                        # the hover→overlay highlight.
+                        "grounding": s.grounding,
                     }
                     for s in items
                 ]