Implement EncryptedBackend for redb

[praveenperera]

XChaCha20-Poly1305 block-level AEAD implementation of redb's StorageBackend trait.

• Implement EncryptedBackend struct wrapping a File, XChaCha20Poly1305 cipher, and logical length
• 64-byte file header: magic (COVE), version, salt, logical length, reserved
• Each 4096-byte logical block stored as 4136 bytes on disk: 24-byte random nonce + 4096-byte ciphertext + 16-byte Poly1305 tag
• Block index included as AAD to prevent block swap attacks
• Implement all StorageBackend trait methods: len(), read(), write(), set_len(), sync_data()
• Partial writes trigger read-decrypt-modify-encrypt-write cycle
• Sparse block handling: all-zero disk blocks return plaintext zeros on read
• Public API: create(path, key, salt), open(path, key), read_salt(path), is_encrypted(path)

Files: rust/src/database/encrypted_backend.rs (new), rust/src/database.rs (add pub mod)