feat(relay): allow HTTP operator listener delivery URLs (#8287)

## Summary

Operator listener configuration currently rejects HTTP URLs, preventing
delivery to sibling services over internal HTTP connections. Accept HTTP
and HTTPS delivery URLs while retaining the existing pubkey,
duplicate-route, and URL restrictions.

Add configuration coverage for internal hostnames and custom ports,
verify signed delivery through the real HTTP client, and include the
listener suite in both unit-test runners. Shared NIP-98 signing and push
gateway code are unchanged.

### Related issue

Related: #7934 refactors operator listener delivery tests. No matching
open issue found.

### Testing

- `cargo test -p buzz-relay --lib operator_listener`: 13 passed,
including signed HTTP delivery and redirect rejection.
- `cargo fmt --all -- --check`, shell syntax, Justfile parsing, and diff
whitespace checks passed.
- `just test`: passed.
- `just ci`: stopped at mobile analysis with an existing undefined
`buzzDevPushAppProfile` in
`mobile/test/features/settings/notification_toggle_test.dart:81`.
Workspace and native desktop formatting/Clippy, desktop frontend checks,
and web checks passed. The mobile file is unchanged by this PR.

Locally verified with a spiked listener client

Signed-off-by: jsibbison-square <jsibbison@squareup.com>

scripts/run-tests.sh

@@ -178,6 +178,9 @@ run_unit_tests() {
   run_test_step "buzz-relay push runtime tests" \
     cargo test -p buzz-relay --lib push_runtime:: -- --nocapture
 
+  run_test_step "buzz-relay operator listener tests" \
+    cargo test -p buzz-relay --lib operator_listener::tests:: -- --nocapture
+
   run_test_step "buzz-relay channel authorization tests" \
     cargo test -p buzz-relay --lib handlers::channel_authz:: -- --nocapture