Update build configuration and improve token handling

- Change CI container image to debian:trixie-slim and set GOPROXY.
- Update Go version to 1.26 in Dockerfile and go.mod.
- Refactor token validation to use SHA-256 hashes instead of plain tokens.
- Add network policy to restrict access to the service.
- Enhance README with new configuration details and usage examples.
- Add tests for new token hash validation logic.

Dockerfile

@@ -1,7 +1,9 @@
-ARG GO_VERSION=1.24
+ARG GO_VERSION=1.26
+ARG GOPROXY_URL=https://gitea.nehmer.net/api/packages/torben/go,https://proxy.golang.org,direct
 
 FROM golang:${GO_VERSION}-bookworm AS build
 WORKDIR /src
+ENV GOPROXY=${GOPROXY_URL}
 
 COPY go.mod ./
 RUN go mod download