feat: speed up cache operations with optimized encryption

Cache load/save operations now complete in milliseconds instead of
hundreds of milliseconds, making transaction syncs noticeably faster
while maintaining full AES-GCM security.

Cargo.lock

@@ -203,6 +203,7 @@ dependencies = [
  "dotenvy",
  "firefly-client",
  "gocardless-client",
+ "hkdf",
  "http",
  "hyper",
  "mockall",
@@ -948,6 +949,15 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc0fef456e4baa96da950455cd02c081ca953b141298e41db3fc7e36b1da849c"
 
+[[package]]
+name = "hkdf"
+version = "0.12.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7b5f8eb2ad728638ea2c7d47a21db23b7b58a72ed6a38256b8a1849f15fbbdf7"
+dependencies = [
+ "hmac",
+]
+
 [[package]]
 name = "hmac"
 version = "0.12.1"