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.

docs/architecture.md

@@ -51,7 +51,7 @@ Two key ideas that explain most of the complexity:
 
 ## 2. Tech stack
 
-**Backend (Go 1.26):**
+**Backend (Go 1.27):**
 
 - Web framework: **Gin** (`gin-gonic/gin`) + sessions (cookie store), gzip.
 - ORM: **GORM** with **SQLite** (default) or **PostgreSQL** (`XUI_DB_TYPE=postgres`).