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/adapters/store/migrate_test.go

@@ -0,0 +1,61 @@
+package store_test
+
+import (
+	"context"
+	"database/sql"
+	"os"
+	"testing"
+
+	"github.com/golang-migrate/migrate/v4"
+	migratepgx "github.com/golang-migrate/migrate/v4/database/pgx/v5"
+	"github.com/golang-migrate/migrate/v4/source/iofs"
+	"github.com/stretchr/testify/require"
+
+	_ "github.com/jackc/pgx/v5/stdlib" // register the "pgx" database/sql driver
+)
+
+// fileMigrator builds a golang-migrate instance from the on-disk migration files
+// (not the embedded FS the production Migrate uses), so a test can step the schema
+// up and down. os.DirFS(".") is rooted at the package dir; the SQL lives under
+// "migrations". Mirrors store.Migrate's construction otherwise.
+func fileMigrator(t *testing.T) *migrate.Migrate {
+	t.Helper()
+	db, err := sql.Open("pgx", dsn)
+	require.NoError(t, err)
+	t.Cleanup(func() { _ = db.Close() })
+
+	drv, err := migratepgx.WithInstance(db, &migratepgx.Config{})
+	require.NoError(t, err)
+	src, err := iofs.New(os.DirFS("."), "migrations")
+	require.NoError(t, err)
+	m, err := migrate.NewWithInstance("iofs", src, "pgx", drv)
+	require.NoError(t, err)
+	t.Cleanup(func() { _, _ = m.Close() })
+	return m
+}
+
+// loginEventsExists reports whether the login_events relation is present.
+func loginEventsExists(t *testing.T) bool {
+	t.Helper()
+	var reg *string
+	require.NoError(t, rawPool(t).QueryRow(context.Background(),
+		`SELECT to_regclass('public.login_events')::text`).Scan(&reg))
+	return reg != nil
+}
+
+// TestMigration010LoginEventsUpDown proves migration 010 is reversible: the down
+// migration drops login_events cleanly and the up migration recreates it. A rotten
+// down migration (forgotten DROP, dangling policy) would fail here rather than in
+// production during a rollback. The test restores the schema to latest before
+// returning so the shared embedded-postgres stays at HEAD for sibling tests.
+func TestMigration010LoginEventsUpDown(t *testing.T) {
+	newStore(t) // ensure the schema is migrated to latest (010 applied)
+	require.True(t, loginEventsExists(t), "login_events must exist at latest migration")
+
+	m := fileMigrator(t)
+	require.NoError(t, m.Steps(-1), "down one migration must drop login_events")
+	require.False(t, loginEventsExists(t), "login_events must be gone after the down migration")
+
+	require.NoError(t, m.Steps(1), "up one migration must recreate login_events")
+	require.True(t, loginEventsExists(t), "login_events must be restored after the up migration")
+}