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/config/config.go

@@ -29,9 +29,10 @@ type GRPC struct {
 }
 
 type Listener struct {
-	Addr          string  `toml:"addr"`
-	ProxyProtocol bool    `toml:"proxy_protocol"`
-	Routes        []Route `toml:"routes"`
+	Addr           string  `toml:"addr"`
+	ProxyProtocol  bool    `toml:"proxy_protocol"`
+	MaxConnections int64   `toml:"max_connections"` // 0 = unlimited
+	Routes         []Route `toml:"routes"`
 }
 
 type Route struct {
@@ -176,6 +177,9 @@ func (c *Config) validate() error {
 		if l.Addr == "" {
 			return fmt.Errorf("listener %d: addr is required", i)
 		}
+		if l.MaxConnections < 0 {
+			return fmt.Errorf("listener %d (%s): max_connections must not be negative", i, l.Addr)
+		}
 		seen := make(map[string]bool)
 		for j := range l.Routes {
 			r := &l.Routes[j]