chore: bump reqwest to 0.13

[dobby-coder]

Summary

Bumps reqwest from 0.12 → 0.13. Closes #11.

Verification

• cargo build
• cargo clippy --all-targets -- -D warnings
• cargo fmt --check
• cargo audit — clean
• Integration tests — green on CI

Reviewer quickstart

git fetch origin && git checkout chore/bump-reqwest-0.13 && cargo build && cargo clippy --all-targets -- -D warnings

[dobby-coder]

Trimmed the PR body: the 'Notes on the major bump' section was over-explanation for a one-line version bump. Per the project's no-justification-paragraphs-for-simple-changes guidance, the diff and CI signal are the explanation. Verification and quickstart kept; substance unchanged.