Epic: HTTP client consolidation

[bug-ops]

Summary

7+ independent reqwest::Client::new() calls across crates with no shared configuration.

Child Issues

• Create zeph_core::http::default_client() with standard config #666 Create zeph_core::http::default_client() with standard config
• Replace all direct Client::new() calls with shared client #667 Replace all direct Client::new() calls with shared client