Build infrastructure for releases

- Makefile: cross-compile to linux/{amd64,arm64}, darwin/{amd64,arm64},
  windows/amd64 into dist/ with SHA256SUMS. Version is stamped via
  -ldflags '-X main.version=...'.
- .gitea/workflows/release.yml: on tag push (v*) the workflow runs
  `make dist`, creates a Gitea release for the tag and uploads every
  artefact from dist/ via the Gitea API.
- main.go: `rb-search version` / --version prints the stamped version.
- README: new "Готовые бинари" section with curl/Invoke-WebRequest
  install snippets and a checksum-verify hint.

main.go

@@ -11,6 +11,9 @@ import (
 	"time"
 )
 
+// version is overridden at build time via -ldflags '-X main.version=...'.
+var version = "dev"
+
 const usage = `rb-search — peek into a RabbitMQ queue without consuming.
 
 Reads N=queue-depth messages with basic.get + manual ack, runs a substring
@@ -72,6 +75,9 @@ func run(args []string) error {
 			return cmdSetDefault(args[1:])
 		case "init":
 			return cmdInit(args[1:])
+		case "version", "--version", "-V":
+			fmt.Println(version)
+			return nil
 		case "help", "-h", "--help":
 			fmt.Print(usage)
 			return nil