Generalize Make overlay toggles to traefik / caddy / observability

[CybotTM]

PR #21 added make enable-bugsink / make enable-sentry (+ disable-* / overlays) so operators don't need to chain -f compose.yml -f examples/compose.X.yml manually. The pattern persists overlay state in .env via COMPOSE_FILE and the bin/ helpers (env-set.sh / compose-file.sh) are generic.

Same UX should be applied to the other three opt-in overlays:

• examples/compose.traefik.yml → make enable-traefik
• examples/compose.caddy.yml → make enable-caddy
• examples/compose.observability.yml → make enable-observability

Each is essentially one Makefile entry that calls ./bin/compose-file.sh add examples/compose.X.yml, plus a matching disable- and a 1-line README table row update.

Source: explicit "Out of scope" item in PR #21 (#21) — "Make targets for traefik / caddy / observability overlays — same pattern, drop in later if the bugsink/sentry pair earns its keep". The pair has now landed, so this is a follow-up.

Acceptance:

• make help lists 4 enable-* + 4 disable-* targets covering all overlays in examples/
• make overlays (already exists) continues to list whichever are active
• Cross-overlay warnings only where they make sense (none for these three — they're independent)

Out of scope: make enable-bugsink / enable-sentry already done.