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/db/seed.go

@@ -18,8 +18,8 @@ func (s *Store) Seed(listeners []config.Listener, fw config.Firewall) error {
 
 	for _, l := range listeners {
 		result, err := tx.Exec(
-			"INSERT INTO listeners (addr, proxy_protocol) VALUES (?, ?)",
-			l.Addr, l.ProxyProtocol,
+			"INSERT INTO listeners (addr, proxy_protocol, max_connections) VALUES (?, ?, ?)",
+			l.Addr, l.ProxyProtocol, l.MaxConnections,
 		)
 		if err != nil {
 			return fmt.Errorf("seeding listener %q: %w", l.Addr, err)