build: bump golang 1.24 → 1.25

go.mod requires >= 1.25.0; previous Dockerfile pinned 1.24 which failed
at `go mod download` with: "go: go.mod requires go >= 1.25.0".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -3,7 +3,7 @@
 #   /tenant-registry  — long-running API server
 #   /migrate          — one-shot schema migrator (Orca init container in prod)
 
-FROM golang:1.24-alpine AS build
+FROM golang:1.25-alpine AS build
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download