ci: add Gitea self-hosted deploy workflow

[Copilot]

Adds a Gitea Actions workflow that auto-deploys the mailserver on push to main using the runner's host Docker socket — no external registry, no artifacts.

Changes

• .gitea/workflows/deploy.yml — new workflow with two steps:
  • git pull via a throwaway alpine/git container mounted at /home/ubuntu/d/mailserver
  • docker compose up -d --build run directly against the host daemon through the mounted socket

- name: Pull latest code
  run: |
    docker run --rm \
      -v /home/ubuntu/d/mailserver:/repo \
      -w /repo \
      alpine/git \
      pull

- name: Rebuild and restart
  run: |
    docker compose -f /home/ubuntu/d/mailserver/docker-compose.yml up -d --build

Requires a self-hosted Gitea runner with /var/run/docker.sock mounted from the host.

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