feat(tag-eval): "keep" records a confirmation so doubts stop resurfacing

"Keep" on a doubted positive was a no-op, so the same confirmed-correct images
came back in "head doubts" every run (operator-flagged: reinforcement keeps
surfacing the same images). Add tag_positive_confirmation (mirror of
tag_suggestion_rejection): keep → POST /images/<id>/tags/<tag_id>/confirm, and
the eval excludes confirmed positives from the doubts list — exactly as rejected
items already drop out of the suggest list. The tag stays a positive either way
(confirmation is a "reviewed" marker, not a training change).

- model TagPositiveConfirmation + migration 0057; confirm endpoint (idempotent).
- tag_eval: _confirmed_ids + exclude from head_doubts_positive examples.
- store.confirmTag + card "keep" calls it.

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

frontend/src/stores/tagEval.js

@@ -47,5 +47,11 @@ export const useTagEvalStore = defineStore('tagEval', () => {
       { body: { tag_id: tagId } })
   }
 
-  return { start, getRun, latest, applyTag, rejectTag, removeTag }
+  // "Keep" — affirm a doubted positive is correct. Records a confirmation so it
+  // stops resurfacing in the doubts list (it stays a positive either way).
+  async function confirmTag(imageId, tagId) {
+    return await api.post(`/api/images/${imageId}/tags/${tagId}/confirm`)
+  }
+
+  return { start, getRun, latest, applyTag, rejectTag, removeTag, confirmTag }
 })