chore(build): bump Go toolchain to 1.27.0

Go 1.27.0 shipped on 2026-08-19. Raise the go directive and the builder
image so Docker and release builds pick it up; every CI job already reads
the version from go.mod, and golangci-lint v2.13.1 release binaries are
themselves built with go1.27.0, so the lint job needs no pin change.

CLAUDE.md

@@ -8,7 +8,7 @@ index, layering rules), read `docs/architecture.md` on demand — do not guess
 file locations when it can answer in one hop.
 
 ## Stack
-- Backend: Go 1.26 (`module github.com/mhsanaei/3x-ui/v3`), Gin, GORM.
+- Backend: Go 1.27 (`module github.com/mhsanaei/3x-ui/v3`), Gin, GORM.
   Runs Xray-core as a managed child process (`internal/xray/process.go`) and
   imports `github.com/xtls/xray-core` for config types + gRPC stats/handler/router
   API. MTProto inbounds run a second managed child — the `mtg-multi` binary