OpenTelemetry span events and missing instrumentation

Enhanced OpenTelemetry observability by adding span events to capture
rich activity data and instrumenting previously uncovered operations.

Span events added:

- activitypub.activity.received: Records full activity JSON and
  verification status (activity, HTTP signatures, LD signatures)
  in the activitypub.inbox span
- activitypub.activity.sent: Records activity JSON and target inbox
  in the activitypub.send_activity span
- activitypub.object.fetched: Records object type and JSON-LD in
  the activitypub.lookup_object span

New spans added:

- docloader.fetch: Tracks document loading, HTTP redirects, and
  final document URLs
- activitypub.verify_key_ownership: Tracks key ownership verification
  with actor ID, key ID, result, and verification method

Also added comprehensive test coverage using a new TestSpanExporter
helper to verify all instrumentation is properly recorded.

Resolves https://github.com/fedify-dev/fedify/issues/323



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