Build queue producer a wire-up

Producer-side build pipeline chyběla celá: nový buildEnqueue.ts s 3-step guard chain (HEAD → markBuildHead → detect → BUILD_QUEUE.send), dispatcher rozšířen v queue.ts o build kind, wire-up v routes/git.ts po addRepoToOwner s vlastním try/catch, BUILD_QUEUE binding v wrangler.jsonc.