feat: Gitea Actions CI + env file + Gitea registry image

- .gitea/workflows/docker.yml: builds and pushes image to
  git.albert-zangerl.com/al/logwatch on every push to main
  and on version tags (v*)
- stack.env.example: template for environment variables
- stack.env: gitignored, lives on docker.lan next to the stack
- portainer-stack.yml: updated to use Gitea registry image
  and env_file reference

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

stack.env.example

@@ -0,0 +1,11 @@
+# ntfy source channel (Watchtower notifications)
+SOURCE_URL=https://ntfy.albert-zangerl.com/albert
+
+# ntfy target channel (important notifications only)
+TARGET_URL=https://ntfy.albert-zangerl.com/wichtig
+
+# Optional: Bearer token for protected ntfy target channel
+# TARGET_TOKEN=
+
+LOG_LEVEL=INFO
+RECONNECT_DELAY=10