refactor file system operations

the most notable improvement is that uploads are now using streams so
they no longer require the entire file to be stored in memory

backend/Cargo.toml

@@ -15,11 +15,13 @@ path = "src/bin/backend/main.rs"
 anyhow = "1.0.98"
 argon2 = "0.5.3"
 axum = { version = "0.8.4", features = ["multipart", "query"] }
-axum_typed_multipart = "0.16.3"
+axum-extra = { version = "0.10.1", features = ["multipart"] }
 base64 = "0.22.1"
+bytes = "1.10.1"
 chrono = "0.4.41"
 derive_more = { version = "2.0.1", features = ["display"] }
 dotenv = "0.15.0"
+futures-util = "0.3.31"
 hex = "0.4.3"
 mime_guess = "2.0.5"
 regex = "1.11.1"