feat(store): add append-only login_events table with forced RLS

Stage-0 usage measurement (VISION/ADR-016): summary_actions captures acts
(watch/skip/save) but not reads. A reader who logs in weekly and clicks
nothing is invisible — for a reading product that return is the signal the
gate ("usage in >=2 distinct weeks") is defined on. login_events records
THAT a user was active, append-only, one row per user per active day.

Per-user isolation via the same GUC-keyed FORCE RLS policy as migration 003.
No FK to users (mirrors summary_actions) — the cascade footgun is handled by
DeleteUser in a later commit. Adds an up/down reversibility test and registers
the table in both truncate helpers.

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

internal/web/handlers_test.go

@@ -72,7 +72,7 @@ func rawPool(t *testing.T) *pgxpool.Pool {
 func truncateAll(t *testing.T, p *pgxpool.Pool) {
 	t.Helper()
 	_, err := p.Exec(context.Background(),
-		`TRUNCATE summary_actions, sink_deliveries, summaries, transcripts, videos, users CASCADE`)
+		`TRUNCATE login_events, summary_actions, sink_deliveries, summaries, transcripts, videos, users CASCADE`)
 	require.NoError(t, err)
 }