refactor: Fix compilation warnings across multiple crates

- Remove unused imports (Filter, Digest, Resource, etc.)
- Prefix unused variables with underscore (_pk, _size)
- Fix snake_case naming for local variables
- Remove unused ArgumentError enum in pmoupnp
- Apply cargo fix suggestions for unused imports

Remaining warnings are async_fn_in_trait style warnings which
would require API breaking changes to address.

pmocache/src/cache_trait.rs

@@ -1,5 +1,4 @@
 use anyhow::Result;
-use sha1::{Digest, Sha1};
 use std::{
     path::{Path, PathBuf},
     sync::Arc,