Multi-backend routing

[ChristianPavilonis]

Summary

• split the multi-backend routing work into a standalone PR
• add backend routing config/build wiring and request-time origin selection
• add routing integration tests, harness support, and user-facing docs

What changed

• add BackendRouter and backend routing settings/types
• merge backends.toml into embedded config at build time
• route publisher requests by host/path before proxying to origin
• add routing integration tests and test-backends support
• document multi-backend routing in the docs site

Verification

• cargo fmt --all -- --check
• cargo test --workspace
• cargo clippy --workspace --all-targets --all-features -- -D warnings

Follow-up

• DataDome server-side validation changes will land in a stacked follow-up PR.