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/store_test.go

@@ -74,7 +74,7 @@ func rawPool(t *testing.T) *pgxpool.Pool {
 func resetDB(t *testing.T, p *pgxpool.Pool) {
 	t.Helper()
 	_, err := p.Exec(context.Background(),
-		`TRUNCATE sink_deliveries, summaries, transcripts, videos, users CASCADE`)
+		`TRUNCATE summary_actions, sink_deliveries, summaries, transcripts, videos, users CASCADE`)
 	require.NoError(t, err)
 }