Fix critical runtime bugs and security issues

[harshdesaii]

Description

Fix frozen timestamps, recursive retry risk, async/sync memory mismatch, replace fake embeddings with real computation and add guidance for secure JWT configuration.

Fixes # (issue)

• Fixed datetime.now() frozen at import time
• Replaced asdict(self) with self.model_dump()
• Fixed recursive retry in router
• Replaced dummy zero embeddings with real pooled embeddings
• Fixed claude image payload encoding(hex -> base64)
• Hardcoded JWT secret/default users removed; env-based JWT setup enforced
• Fixed async/sync mismatch in memory layer