Rename EncryptedBackend → CoveEncryptedBackend

[praveenperera]

Rename the struct and module for clarity now that we're adding cloud upload hooks.

• Rename EncryptedBackend → CoveEncryptedBackend in encrypted_backend.rs
• Add path: PathBuf field to the struct (needed for change detection in next issue)
• No data migration needed — on-disk format is identified by magic bytes b"COVE" + version byte, not the Rust struct name
• Purely a code change (rename + add field)

Files: rust/src/database/encrypted_backend.rs, rust/src/database.rs, rust/src/database/wallet_data.rs