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>

internal/server/server.go

@@ -34,6 +34,7 @@ type ListenerState struct {
 	ID                int64 // database primary key
 	Addr              string
 	ProxyProtocol     bool
+	MaxConnections    int64 // 0 = unlimited
 	routes            map[string]RouteInfo // lowercase hostname → route info
 	mu                sync.RWMutex
 	ActiveConnections atomic.Int64
@@ -41,6 +42,13 @@ type ListenerState struct {
 	connMu            sync.Mutex
 }
 
+// SetMaxConnections updates the connection limit at runtime.
+func (ls *ListenerState) SetMaxConnections(n int64) {
+	ls.mu.Lock()
+	defer ls.mu.Unlock()
+	ls.MaxConnections = n
+}
+
 // Routes returns a snapshot of the listener's route table.
 func (ls *ListenerState) Routes() map[string]RouteInfo {
 	ls.mu.RLock()
@@ -93,10 +101,11 @@ func (ls *ListenerState) lookupRoute(hostname string) (RouteInfo, bool) {
 
 // ListenerData holds the data needed to construct a ListenerState.
 type ListenerData struct {
-	ID            int64
-	Addr          string
-	ProxyProtocol bool
-	Routes        map[string]RouteInfo // lowercase hostname → route info
+	ID             int64
+	Addr           string
+	ProxyProtocol  bool
+	MaxConnections int64
+	Routes         map[string]RouteInfo // lowercase hostname → route info
 }
 
 // Server is the mc-proxy server. It manages listeners, firewall evaluation,
@@ -116,11 +125,12 @@ func New(cfg *config.Config, fw *firewall.Firewall, listenerData []ListenerData,
 	var listeners []*ListenerState
 	for _, ld := range listenerData {
 		listeners = append(listeners, &ListenerState{
-			ID:            ld.ID,
-			Addr:          ld.Addr,
-			ProxyProtocol: ld.ProxyProtocol,
-			routes:        ld.Routes,
-			activeConns:   make(map[net.Conn]struct{}),
+			ID:             ld.ID,
+			Addr:           ld.Addr,
+			ProxyProtocol:  ld.ProxyProtocol,
+			MaxConnections: ld.MaxConnections,
+			routes:         ld.Routes,
+			activeConns:    make(map[net.Conn]struct{}),
 		})
 	}
 
@@ -229,6 +239,13 @@ func (s *Server) serve(ctx context.Context, ln net.Listener, ls *ListenerState)
 			continue
 		}
 
+		// Enforce per-listener connection limit.
+		if ls.MaxConnections > 0 && ls.ActiveConnections.Load() >= ls.MaxConnections {
+			conn.Close()
+			s.logger.Debug("connection limit reached", "addr", ls.Addr, "limit", ls.MaxConnections)
+			continue
+		}
+
 		s.wg.Add(1)
 		ls.ActiveConnections.Add(1)
 		go s.handleConn(ctx, conn, ls)