feat(store): default auto_summarize ON for new users (ADR-018)

Migration 011 flips the auto_summarize column default to TRUE and brings
existing rows (maintainer + current registrations) along. Onboarded friends
now get zero-friction discovery: scheduled discovery (ADR-018) both discovers
AND summarizes new videos, so a user's list fills and summarizes itself
instead of presenting an empty list of manual Summarize buttons.

Safe only because the process-wide caption-fetch rate gate (ADR-014 item 2,
prior commit) now exists — auto + scheduled + multi-user would otherwise
self-inflict 429s every cycle. The down migration reverts the default but
intentionally leaves existing rows as-is (no surprise manual regression on
rollback). RegisterUser already lets the column default drive the value, so
no app change is needed; the account-page manual toggle still works.

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

internal/adapters/store/summarize_mode_test.go

@@ -45,6 +45,27 @@ func TestAutoSummarizeRoundTripDefaultsFalse(t *testing.T) {
 	require.False(t, got, "set back to manual round-trips")
 }
 
+func TestRegisteredUserDefaultsAutoSummarizeOn(t *testing.T) {
+	ctx := context.Background()
+	s := newStore(t)
+	resetDB(t, rawPool(t))
+
+	// A real registration creates the users row, so the column default (migration
+	// 011: TRUE) drives the mode — onboarded friends get auto out of the box.
+	id, err := s.RegisterUser(ctx, subjectA, "Alice")
+	require.NoError(t, err)
+
+	got, err := s.GetAutoSummarize(ctx, id)
+	require.NoError(t, err)
+	require.True(t, got, "new registrations default to auto-summarize (ADR-018)")
+
+	// The account-page toggle still works: a user can switch to manual.
+	require.NoError(t, s.SetAutoSummarize(ctx, id, false))
+	got, err = s.GetAutoSummarize(ctx, id)
+	require.NoError(t, err)
+	require.False(t, got, "the manual toggle still flips it off")
+}
+
 func TestRequestSummarizeSetsFlag(t *testing.T) {
 	ctx := context.Background()
 	s := newStore(t)