Conditionally register Key auth endpoint based on DB pool availability

[Copilot]

The /sign/key endpoint with IrmaAuthType::Key was always registered, causing middleware initialization to fail when db_pool is None (lines 305-309 in irma.rs). Since database configuration is optional, this prevented server startup in valid configurations.

Changes:

• Refactored IRMA scope registration to use mutable variable pattern
• Conditionally register API-key /sign/key service only when db_pool.is_some()
• JWT authentication fallback remains unconditionally available

Before:

.service(
    resource("/sign/key")
        .wrap({
            let auth = IrmaAuth::new(irma.clone(), IrmaAuthType::Key);
            if let Some(ref pool) = db_pool {
                auth.with_db_pool(Data::clone(pool))
            } else {
                auth  // <- Still fails at middleware init
            }
        })
)

After:

let mut irma_scope = scope("/{_:(irma|request)}")
    .service(/* other services */);

if let Some(ref pool) = db_pool {
    irma_scope = irma_scope.service(
        resource("/sign/key")
            .wrap(IrmaAuth::new(irma, IrmaAuthType::Key)
                .with_db_pool(Data::clone(pool)))
    );
}

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