Honor credential reusable flag (#2157)

- propagate reusable through credential storage, CLI, RPC, routing, and tests
- enforce reusable=false owner election with current topology
- preserve proof-backed groups when refreshing credential ACL groups

easytier/src/proto/api_instance.proto

@@ -318,6 +318,7 @@ message GenerateCredentialRequest {
   int64 ttl_seconds = 4;        // must be > 0: credential TTL in seconds (0 / omitted is invalid)
   optional string credential_id = 5; // optional: user-specified credential id, reused if already exists
   InstanceIdentifier instance = 6;   // target network instance
+  optional bool reusable = 7;   // default true: allow multiple peers to reuse this credential
 }
 
 message GenerateCredentialResponse {
@@ -344,6 +345,7 @@ message CredentialInfo {
   bool allow_relay = 3;
   int64 expiry_unix = 4;
   repeated string allowed_proxy_cidrs = 5;
+  optional bool reusable = 6;
 }
 
 message ListCredentialsResponse {