Add per-listener connection limits

Configurable maximum concurrent connections per listener. When the
limit is reached, new connections are closed immediately after accept.
0 means unlimited (default, preserving existing behavior).

Config: Listener gains max_connections field, validated non-negative.

DB: Migration 3 adds listeners.max_connections column.
UpdateListenerMaxConns method for runtime changes via gRPC.
CreateListener updated to persist max_connections on seed.

Server: ListenerState/ListenerData gain MaxConnections. Limit checked
in serve() after Accept but before handleConn — if ActiveConnections
>= MaxConnections, connection is closed and the accept loop continues.
SetMaxConnections method for runtime updates.

Proto: SetListenerMaxConnections RPC added. ListenerStatus gains
max_connections field. Generated code regenerated.

gRPC server: SetListenerMaxConnections implements write-through
(DB first, then in-memory update). GetStatus includes max_connections.

Client: SetListenerMaxConnections method, MaxConnections in
ListenerStatus.

Tests: DB CRUD and UpdateListenerMaxConns, server connection limit
enforcement (accept 2, reject 3rd, close one, accept again), gRPC
SetListenerMaxConnections round-trip with DB persistence, not-found
error handling.

Also updates PROJECT_PLAN.md with phases 6-8 and PROGRESS.md with
tracking for the new features.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

proto/mc_proxy/v1/admin.proto

@@ -17,6 +17,9 @@ service ProxyAdminService {
   rpc AddFirewallRule(AddFirewallRuleRequest) returns (AddFirewallRuleResponse);
   rpc RemoveFirewallRule(RemoveFirewallRuleRequest) returns (RemoveFirewallRuleResponse);
 
+  // Connection limits
+  rpc SetListenerMaxConnections(SetListenerMaxConnectionsRequest) returns (SetListenerMaxConnectionsResponse);
+
   // Status
   rpc GetStatus(GetStatusRequest) returns (GetStatusResponse);
 }
@@ -90,11 +93,19 @@ message RemoveFirewallRuleResponse {}
 
 // Status
 
+message SetListenerMaxConnectionsRequest {
+  string listener_addr = 1;
+  int64 max_connections = 2;  // 0 = unlimited
+}
+
+message SetListenerMaxConnectionsResponse {}
+
 message ListenerStatus {
   string addr = 1;
   int32 route_count = 2;
   int64 active_connections = 3;
   bool proxy_protocol = 4;
+  int64 max_connections = 5;
 }
 
 message GetStatusRequest {}