fix: emit webhook delivery headers

[rickcrawford]

Summary

• Closes WOR-88 by adding Sbproxy-Subscription-Id, Sbproxy-Delivery-Id, and Sbproxy-Attempt to outbound webhook delivery attempts.
• Adds a stable subscription_id field to Subscription and generates a fresh delivery ULID for every retry attempt.
• Extends notifier tests and updates the changelog.

Test plan

• RED: cargo test -p sbproxy-observe retry_then_deadletter failed because Sbproxy-Attempt was missing
• GREEN: cargo test -p sbproxy-observe retry_then_deadletter
• cargo test -p sbproxy-observe notify::tests::dispatch_signs_with_ed25519
• cargo test -p sbproxy-observe --lib
• cargo fmt --all -- --check
• git diff --check
• IDE lints: no errors for crates/sbproxy-observe/src/notify.rs

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