Split ELF API route contract modules

[yvette-carlisle]

Summary

• split ELF API OpenAPI contract wiring out of routes.rs into routes/contract.rs
• split the admin viewer handler into routes/viewer.rs
• replace remaining #[path] route child-module declarations in routes.rs, routes/support.rs, and routes/types.rs with normal Rust modules
• add /v2/graph/report to the generated OpenAPI contract and assert it in the contract test

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo test -p elf-api --test http contract::openapi_json_route_serves_generated_contract
• cargo make check
• read-only skeptic review: no blockers