merge: clear the clippy backlog and unify lock helpers (D1-warnings, D3)

51 clippy warnings -> 0, with 8 justified #[allow]s (IPC arity, specta wire
types, and the 9 test-only await-holding-lock sites). 27 raw lock calls moved to
the poison-tolerant helpers - all of them test code; production was already
clean.

Caught a non-neutral clippy --fix: removing the redundant 'use hostname;' in
credentials.rs orphaned its #[cfg(target_os = "linux")] onto SERVICE_NAME,
which would have cfg'd the constant out of every non-Linux build. Compiles clean
on Linux, so only Windows/macOS CI would have caught it.

src-tauri/src/commands/repository.rs

@@ -67,6 +67,10 @@ pub struct RepositoryManagerWrapper(pub RepositoryManager);
 /// Returns a handle (UUID) for accessing the repository
 #[tauri::command]
 #[specta::specta]
+// Four of the eight arguments are Tauri `State<'_, _>` injections, not caller
+// input. Folding the remaining four into a struct would change the IPC contract
+// and the generated TypeScript for no readability gain.
+#[allow(clippy::too_many_arguments)]
 pub async fn repository_create(
     manager: State<'_, RepositoryManagerWrapper>,
     player: State<'_, crate::commands::player::PlayerStateWrapper>,
@@ -1099,7 +1103,6 @@ mod tests {
         let handle = format!("{}", uuid);
         // UUID should convert to a non-empty string
         assert!(!handle.is_empty());
-        assert!(handle.len() > 0);
     }
 
     #[test]