New utils/user-keys.ts: one random 32-byte DEK per user, wrapped
AES-256-GCM under an HKDF key derived from the system ENCRYPTION_KEY
(per-user info string + AAD binding, versioned v3 wrap format stored in
settings). Synchronous unwrap-on-demand with a 15-minute cache so the
existing DataCrypto facade keeps its sync call sites. Not wired up yet.
LukeGus