ci: deploy native binary directly to LXC via SSH (drop Docker/registry)

Switch the runtime from a Docker image to a systemd service running the
native binary on the LXC host. The CI still uses Docker for the build
environment (maven:3.9.6-eclipse-temurin-21), but stops at producing the
static native binary.

Pipeline changes:
- Drop docker.io, docker-buildx, docker buildx, docker push, registry.
- Drop Dockerfile, compose.yaml, .dockerignore (no longer needed).
- Build native binary in CI container, SCP to LXC, run deploy script.
- Deploy script stops the service, swaps the binary, starts it, hits
  /q/health/live to verify.

LXC one-time setup (manual, run on the host):
- useradd runner (UID 1001)
- mkdir /opt/shot-crafter-calculator/{data,keys,deploy}
- copy RSA JWT keys into keys/
- install /etc/systemd/system/shot-crafter-calculator.service
- install /usr/local/bin/deploy-shot-crafter-calculator.sh
- useradd deployer + ssh keypair for the CI
- store DEPLOY_SSH_KEY secret in Gitea

Bootstrap the first deploy manually with scp + ssh before relying on CI.

.dockerignore

@@ -1,8 +0,0 @@
-.git
-target
-node_modules
-data
-.mvn
-*.md
-.gitignore
-.gitea