feat: Gitea Actions CI/CD pipeline with container registry builds

- .gitea/workflows/docker.yml: builds and pushes image to
  devops.cloudhost.es/malin/paste.es on every push to main,
  tags as :latest and :<short-sha>, uses GHA layer cache
- docker-compose.yml: updated to pull pre-built image from Gitea
  container registry instead of building locally
- docker-compose.build.yml: new override file for local dev builds
  (docker compose -f docker-compose.yml -f docker-compose.build.yml up --build)
- docker-compose.runner.yml: act_runner service — mount docker socket,
  auto-registers with RUNNER_REGISTRATION_TOKEN from .env
- .env.example: added RUNNER_REGISTRATION_TOKEN section
- README.md: document CI/CD setup, runner registration, and updated
  quick-start to use registry pull workflow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -117,3 +117,12 @@ HEMMELIG_ANALYTICS_ENABLED=false
 # -----------------------------------------------------------------------------
 
 # (Los webhooks se configuran desde el panel de administración en /dashboard/instance)
+
+
+# -----------------------------------------------------------------------------
+# GITEA ACTIONS RUNNER — Solo necesario si ejecutas el runner en este servidor
+# Obtén el token en: https://devops.cloudhost.es/Malin/paste.es/settings/actions/runners
+# Luego arranca con: docker compose -f docker-compose.runner.yml up -d
+# -----------------------------------------------------------------------------
+
+# RUNNER_REGISTRATION_TOKEN=