chore: bump Go toolchain to 1.26.3 (#2)

Pin Go version to 1.26.3 across go.mod, CI workflows, and Dockerfiles
for canonical alignment with the rest of the luxfi/* stack.

Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM golang:1.26-alpine AS builder
+FROM golang:1.26.3-alpine AS builder
 
 # Install build dependencies
 RUN apk add --no-cache git make