chore(ci): add cargo fmt and clippy pre-commit hooks for backend/

[AnnabelJoe]

Summary

Closes #156

Adds explicit cargo fmt and cargo clippy pre-commit hooks via doublify/pre-commit-rust for both Rust crates under backend/.

Changes to .pre-commit-config.yaml

New repo block added using https://github.com/doublify/pre-commit-rust (v1.0):

Hook	Target	Behaviour
fmt	backend/indexer	Fails if code is not formatted (--check)
fmt	backend/oracle	Fails if code is not formatted (--check)
clippy	backend/indexer	Fails on any warning (-D warnings)
clippy	backend/oracle	Fails on any warning (-D warnings)

All four hooks run on commit and manual stages.

The existing local quality-gate script (scripts/pre-commit-hook.sh) is retained — it additionally runs cargo test --lib which the new hooks do not cover.

Install / Update Hooks Locally

After pulling this branch, run:

bash

Install pre-commit if not already present

pip install pre-commit

Install/update the hook environments (downloads doublify/pre-commit-rust)

pre-commit install
pre-commit install --hook-type pre-commit

Run all hooks manually against the full repo to verify

pre-commit run --all-files

To run only the Rust hooks manually:
bash
pre-commit run fmt --all-files
pre-commit run clippy --all-files

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[ayomideadeniran]

Thanks for the PR! This is currently under review and I will get back to you shortly.