refactor: remove dead contract artifacts

[h1065153539-create]

Summary

• Removes quicklendx-contracts/src/lib.rs.bak, the stale backup copy of the contract entrypoint.
• Removes quicklendx-contracts/src/events_escrow_stub.rs, which has no remaining module/include/path references.
• Leaves scratch_events.rs and backup_v1.rs in place because the reference check shows they are still wired through #[cfg(all(test, feature = "legacy-tests"))] mod scratch_events; and the V1 backup compatibility path/tests.

Closes #1341

Reference check

• rg -n "events_escrow_stub|lib\.rs\.bak|scratch_events|backup_v1" quicklendx-contracts/src quicklendx-contracts/Cargo.toml .github -g "!quicklendx-contracts/cargo_test*.log"
• No remaining references to events_escrow_stub or lib.rs.bak after removal.
• Remaining references are limited to scratch_events and backup_v1, so those files are intentionally retained per the issue guidance to leave referenced files in place.

Validation

• git diff --check

Existing validation blockers

• cargo fmt --all -- --check reports broad pre-existing formatting drift across the contracts crate; I did not apply formatting to avoid unrelated churn.
• cargo build reaches the crate and currently fails on a pre-existing src/health.rs error: crate::invoice::InvoiceStorage is a private import and get_total_invoice_count is not found.
• cargo test --no-run is additionally blocked locally by the Windows GNU toolchain missing dlltool.exe while compiling getrandom test dependencies.
• cargo clippy --all-targets --all-features could not run because the local E: toolchain does not have the clippy component installed.

[Baskarayelu]

good cleanup - removing the dead contract artifacts trims the crate. merging 🧹