feat(auth): server-side sessions and current password for account changes (FLAW-30, FLAW-31)

- Every login is a session (device, IP, last use); the token's jti is its ID.
  Sessions end on logout, per device from the profile, or after 30 idle days.
- Tokens older than a day are renewed via the X-Session-Token header.
- Changing password or login e-mail needs the current password; a new
  password logs out all other devices and mails the old address.
- Demo reset keeps the demo visitors' sessions.
- revoked_tokens is replaced by sessions (migration 4); everyone is logged
  out once after the upgrade.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/migrations.go

@@ -24,6 +24,7 @@ type migration struct {
 
 var migrations = []migration{
 	{3, "Basis-Schema v0.4.1", (*Store).migrateBaseline},
+	{4, "Sitzungen", (*Store).migrateSessions},
 }
 
 func (s *Store) migrate() error {