Mastodon interoperability smoke tests (non-strict)

[sij411]

Summary

Add automated E2E smoke tests that verify Fedify's interoperability with Mastodon over HTTP with signature verification skipped. This is the foundational smoke test lane that validates ActivityPub message formation and processing.

Parent issue

Sub-issue of #481 (Interoperability smoke test suite).

What it covers

• Fedify test harness app with backdoor APIs for test orchestration
• Docker Compose setup with Mastodon (web, sidekiq, postgres, redis)
• CI orchestrator running 6 scenarios:
  1. Mastodon → Fedify (Follow)
  2. Fedify → Mastodon (Follow)
  3. Fedify → Mastodon (Create Note)
  4. Mastodon → Fedify (Reply)
  5. Mastodon → Fedify (Unfollow)
  6. Fedify → Mastodon (Unfollow)
• Provisioning script with DB pre-registration of the Fedify account
• skipSignatureVerification: true for simplicity

Implementation

PR #598