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>

alembic/versions/0057_tag_positive_confirmation.py

@@ -0,0 +1,40 @@
+"""tag_positive_confirmation: operator-affirmed correct positives (#1130)
+
+Mirror of tag_suggestion_rejection. "Keep" on a doubted positive records here so
+the eval's doubts list stops resurfacing confirmed-correct images every run.
+
+Revision ID: 0057
+Revises: 0056
+Create Date: 2026-06-28
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0057"
+down_revision: Union[str, None] = "0056"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "tag_positive_confirmation",
+        sa.Column(
+            "image_record_id", sa.Integer(),
+            sa.ForeignKey("image_record.id", ondelete="CASCADE"), primary_key=True,
+        ),
+        sa.Column(
+            "tag_id", sa.Integer(),
+            sa.ForeignKey("tag.id", ondelete="CASCADE"), primary_key=True, index=True,
+        ),
+        sa.Column(
+            "confirmed_at", sa.DateTime(timezone=True), nullable=False,
+            server_default=sa.func.now(),
+        ),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("tag_positive_confirmation")