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/migrations.go

@@ -14,6 +14,7 @@ type migration struct {
 var migrations = []migration{
 	{1, "create_core_tables", migrate001CreateCoreTables},
 	{2, "add_proxy_protocol_and_l7_fields", migrate002AddL7Fields},
+	{3, "add_listener_max_connections", migrate003AddListenerMaxConnections},
 }
 
 // Migrate runs all unapplied migrations sequentially.
@@ -110,3 +111,8 @@ func migrate002AddL7Fields(tx *sql.Tx) error {
 	}
 	return nil
 }
+
+func migrate003AddListenerMaxConnections(tx *sql.Tx) error {
+	_, err := tx.Exec(`ALTER TABLE listeners ADD COLUMN max_connections INTEGER NOT NULL DEFAULT 0`)
+	return err
+}