fix(application-autoscaling): persist state across restart; fix example compose healthcheck

[vieiralucas]

Summary

Bug-hunt 2026-06-25.

• Application Auto Scaling persistence (Tier-4 MEDIUM) — scalable targets, scaling policies, scheduled actions, and tags were never persisted: ApplicationAutoScalingService had no snapshot store/hook, so everything vanished on restart in persistent mode while its direct-API siblings survived (it was added recently in test(tfacc): wire application-autoscaling + fix ECS service/cluster delete fidelity #1944 and missed the persistence sweep). Added the standard per-service snapshot pattern (mirroring wafv2): a versioned ApplicationAutoScalingSnapshot, with_snapshot_store / snapshot_hook / save_snapshot, a central save after any successful mutating action, and load-on-boot + the CFN hook wiring in main.rs. The tuple-keyed state maps (TargetKey/PolicyKey/ScheduledKey) can't serialize as JSON object keys, so they round-trip through a map_as_pairs serde adapter (the tuple-key serde gotcha from the persistence sweep).
• Example compose healthcheck (Tier-5 MEDIUM) — examples/docker-compose.yml used curl, which the slim published image doesn't ship, so the example container reported unhealthy forever. Switched to the built-in fakecloud healthcheck probe (matching the root compose file).

Test plan

• Added a round-trip test: RegisterScalableTarget writes a snapshot that deserializes back with the target present (proves the tuple-key serde adapter + save path).
• cargo test -p fakecloud-application-autoscaling — 15 passed.
• Server bin builds; cargo clippy -p fakecloud-application-autoscaling and -p fakecloud --bin fakecloud clean; cargo fmt applied.

Surface check

• No new public API / introspection surface.
• New persisted snapshot file application-autoscaling/snapshot.json follows the existing per-service layout; schema versioned (v1) with forward-compat guard, like the other services.
• ApplicationAutoScalingAccounts/AccountState gained Clone + the map_as_pairs serde adapter; backward-compatible (new on-disk file only written in persistent mode).

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Summary by cubic

Persist Application Auto Scaling state across restarts and fix the example compose healthcheck so containers report healthy. This prevents state loss for scalable targets, policies, schedules, and tags in persistent mode.

• Bug Fixes

  • Persist fakecloud-application-autoscaling state using a versioned snapshot (v1); save after successful mutating actions and load on boot from application-autoscaling/snapshot.json (tuple-key maps serialize via map_as_pairs).
  • Wire snapshot load and CloudFormation snapshot hook in fakecloud-server so CFN-triggered saves also persist.
  • Update examples/docker-compose.yml healthcheck to use fakecloud healthcheck instead of curl.

• Dependencies

  • Add fakecloud-persistence to fakecloud-application-autoscaling.

^{Written for commit f654b11. Summary will update on new commits.}