Split API docs L0 search route

[yvette-carlisle]

Summary

• Move the API docs L0 search inner implementation from routes/docs.rs into routes/docs/search_l0.rs.
• Keep docs_search_l0, admin_docs_search_l0, and their #[utoipa::path] annotations in docs.rs.
• Preserve payload validation, status normalization, temporal bounds checks, query length checks, and service request construction.

Validation

• cargo make fmt-rust
• cargo make check-rust
• cargo make lint-vstyle
• cargo make lint-rust
• cargo make test-rust (370 passed, 92 skipped)
• cargo make check (docs/fmt/check/clippy/vstyle/nextest; 370 passed, 92 skipped)
• cargo make test-rust-integration (92 passed, 370 skipped)
• cargo make check-trace-gate
• git diff --check
• git diff --cached --check
• Fresh read-only staged skeptic review: pass, no objections

Cleanup

• Trace-gate Postgres was run under isolated elf-trace-gate compose project and removed with down -v --remove-orphans.
• Generated trace gate files were deleted before commit.