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/components/settings/TagEvalCard.vue

@@ -247,7 +247,7 @@ async function act(c, it, dir, verdict) {
   if (dir === 'suggest' && verdict === 'yes') { call = store.applyTag(it.id, c.tag_id); label = 'tagged' }
   else if (dir === 'suggest' && verdict === 'no') { call = store.rejectTag(it.id, c.tag_id); label = 'rejected' }
   else if (dir === 'doubts' && verdict === 'no') { call = store.removeTag(it.id, c.tag_id); label = 'removed' }
-  else { acted.value[key] = 'kept'; return }   // doubt + yes = keep, no write
+  else { call = store.confirmTag(it.id, c.tag_id); label = 'kept' }   // doubt + yes = keep (confirm)
   try {
     await call
     acted.value[key] = label