Persist Oracle dashboard sessions (remove in-memory auth store in production)

[adhamhaithameid]

Problem

Oracle logs show the warning that sessions are in-memory and not persisted across restarts.

Why it matters

• admin sessions reset on restart/deploy
• no shared session state for multi-instance scaling
• weaker ops stability for dashboard auth

Scope

• configure persistent session store via POSTGRES_DSN
• add startup production guard when persistent session backend is missing
• document migration + rollback

Acceptance

• Oracle restarts keep valid sessions within TTL
• no in-memory session warning in production logs
• tests + deployment docs updated