Add User Authentication and Session Recording

[pAulseperformance]

Epic: Phase 2 Architecture

Implement Better Auth to assign stable peer_ids, prevent impersonation, and allow users to save or record their jam sessions to a persistent D1/R2 store.

Acceptance Criteria:

• Better Auth Unified Worker pattern implemented.
• Users can log in/sign up.
• Only authenticated users can save presets or sequences.