feat(backup): encrypt backup envelopes at rest with a configurable KEK

Splits backup_envelope.rs into a module (types, read, write, crypto) and
adds an optional AES-256-GCM encryption layer for backup envelopes. A new
`BackupEncryptionSettings` config section points to the 32-byte KEK file.
When the KEK is present the orchestrator calls `finalize_encrypted` and
restore auto-detects the envelope version (1 = plain, 2 = encrypted) to
dispatch to the appropriate parser. If no KEK is configured a per-backup
warning is emitted rather than silently producing a plaintext archive.

The backup KEK is loaded at startup into `ServerState::backup_kek` and a
warning is logged when it shares the same path as the WAL KEK.

Author: farhan-syah

nodedb-types/Cargo.toml

@@ -26,6 +26,9 @@ nodedb-codec = { workspace = true }
 crc32c = { workspace = true }
 roaring = { workspace = true }
 bytemuck = { workspace = true }
+aes-gcm = { workspace = true }
+getrandom = { workspace = true }
+sha2 = { workspace = true }
 
 [dev-dependencies]
 toml = { workspace = true }