Replace standalone md5 crate with RustCrypto md-5

The standalone `md5` (0.8.0) crate is replaced with `md-5` (0.10) from
the RustCrypto ecosystem, which shares `digest v0.10` with sha2, argon2,
blake2 and other crates already in the dependency tree — eliminating one
redundant implementation.

https://claude.ai/code/session_01V23pGpfNw5ujZvtwRFG6qy

Cargo.toml

@@ -38,7 +38,7 @@ dotenvy = "0.15.7"
 moka = { version = "0.12", features = ["future", "sync"] }
 http-range-header = "0.4"
 image = { version = "0.25", default-features = false, features = ["jpeg", "png", "gif", "webp"] }
-md5 = "0.8.0"
+md-5 = "0.10"
 sha2 = "0.10.9"
 blake3 = { version = "1.8.3", features = ["rayon"] }
 hex = "0.4.3"