feat(store): persist summary actions (watch/skip/save) — Stage-0 metric

The summary_actions table records the maintainer's act on a summary, the
column that makes the Stage-0 headline test ("acts on >=1 summary") queryable
(ui-spec.md §5, ADR-011). This is the gate lanes B/C build on.

- Migration 002: summary_actions (id, user_id, video_id TEXT, action, acted_at)
  with a CHECK on action IN ('watched','skipped','saved') and a UNIQUE
  (user_id, video_id, action). Per-user isolation: every row carries user_id.
- New actions.go: SetAction (idempotent, atomic watched<->skipped mutual
  exclusion in one tx; saved independent), ClearAction, ActionsFor for the list
  view, plus Go-side action validation.
- reads.go: additive SummaryRow.Actions, populated by composing ActionsFor
  (Go-side, not a SQL join — summaries.video_id is UUID, actions.video_id TEXT).
- embedded-postgres tests: set/clear, mutual exclusion, saved coexistence,
  idempotency, invalid rejection, user scoping, read-view surfacing.

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

internal/adapters/store/reads.go

@@ -37,6 +37,7 @@ type SummaryRow struct {
 	AIModel      string
 	FallbackUsed bool
 	CreatedAt    time.Time
+	Actions      []string // current active actions for this video; nil when none
 }
 
 // selectSummary is the shared projection for both reads. videos is LEFT JOINed
@@ -87,6 +88,9 @@ func (s *Store) ListSummaries(ctx context.Context, userID string, limit int) ([]
 	if err := rows.Err(); err != nil {
 		return nil, fmt.Errorf("store: iterate summaries: %w", err)
 	}
+	if err := s.attachActions(ctx, userID, out); err != nil {
+		return nil, err
+	}
 	return out, nil
 }
 
@@ -113,7 +117,11 @@ func (s *Store) GetSummaryByVideo(ctx context.Context, userID, videoID string) (
 	if err != nil {
 		return nil, err
 	}
-	return &row, nil
+	holder := []SummaryRow{row}
+	if err := s.attachActions(ctx, userID, holder); err != nil {
+		return nil, err
+	}
+	return &holder[0], nil
 }
 
 // scanSummaryRow reads one row in the selectSummary column order. published_at is