refactor: extract request context utilities into dedicated package

Move ClaimsFromContext and MustUserID helpers from httpserver to new requestctx package for better separation of concerns. Update all imports across auth, device, policy, and user handlers. Fix Dockerfile to copy go.sum and run go mod tidy before download.

backend/Dockerfile

@@ -1,8 +1,10 @@
 FROM golang:1.23-alpine AS builder
 WORKDIR /src
 COPY go.mod ./
-RUN go mod download
+COPY go.sum* ./
 COPY . .
+RUN go mod tidy
+RUN go mod download
 RUN CGO_ENABLED=0 GOOS=linux go build -o /out/nexavpn-api ./cmd/api
 
 FROM alpine:3.21