🤖 Notify reprocessing after HTTP envelope import

[dapplion]

Links audit/review artifact: #100

Implements GLOAS-05 from the audit:

• HTTP execution-payload-envelope import now sends PayloadEnvelopeImported after a successful import.
• The notification follows the gossip path ordering: recompute head, then release attestations waiting on the payload envelope.
• Notification failure is logged instead of turning a successful HTTP import into an API error.

Validation run locally:

• cargo fmt --package http_api
• cargo check -p http_api
• git diff --check