implement Double Ratchet / Ephemeral DH for Perfect Forward Secrecy (PFS) in private chats

[itsVentie]

Description

Currently, Liora uses a static X25519 shared secret derived directly from long-term Ed25519 identity seeds. To ensure Perfect Forward Secrecy (PFS), we need to implement a key-ratcheting mechanism so that the compromise of long-term keys does not expose past session histories.

Todo

• Implement an ephemeral X25519 Diffie-Hellman handshake executed at the start of every new chat session.
• Create a KDF (Key Derivation Function) chain using HKDF-SHA256 to update symmetric AES-GCM keys on every sent/received message (Message Ratchet).
• Add tracking for out-of-order messages to store skipped keys in the local SQLite vault securely (with an explicit retention/expiration policy).
• Update the message processing layer in backend/crypto/ to reject payloads encrypted with stale keys.