Fixed race condition

Bug: https://github.com/cashubtc/cdk/actions/runs/15683152414/job/44190084378?pr=822#step:5:19212

Reason: a race condition between removing proofs while melting and the quote states being updated.

Solution:

1. Error on duplicate proofs
2. Read quote when updating to avoid race conditions and rollbacks

Real solution: A transaction trait in the storage layer. That is coming next

crates/cdk-common/src/database/mod.rs

@@ -22,6 +22,11 @@ pub enum Error {
     /// Database Error
     #[error(transparent)]
     Database(Box<dyn std::error::Error + Send + Sync>),
+
+    /// Duplicate entry
+    #[error("Duplicate entry")]
+    Duplicate,
+
     /// DHKE error
     #[error(transparent)]
     DHKE(#[from] crate::dhke::Error),