Address clippy warnings

# Project Improvement

A large number of clippy warnings are emitted for the crates under this project. We should resolve these warnings and put in place CI jobs to prevent introduction of clippy warnings with any incoming PRs.

The majority of the warnings can be addressed without introducing a breaking change. There are a few warnings that require a breaking change which we can defer to a later point so we can make a conscious decision about it.

I am using the the [cargo-semver-checks](https://crates.io/crates/cargo-semver-checks) crate to verify that the clippy suggestions do not introduce any breaking changes.

# Tracking pull requests
## oxide-auth
- [x] borrow_deref_ref - #209
- [x] derivable_impls
- #209
- #223
- [x] doc_lazy_continuation - #209
- [x] doc_overindented_list_items - #209
- [x] err_expect - #223
- [x] extra_unused_lifetimes - #209
- [x] from_over_into - #209
- [x] get_first - #209
- [ ] large_enum_variant - #234
- [x] len_zero - #209
- [x] let_unit_value - #223
- [x] manual_range_contains - #209
- [x] match_as_ref - #209
- [x] mismatched_lifetime_syntaxes - #209
- [x] missing_safety_doc - #209
- [x] needless_borrow - #209
- [x] needless_borrowed_reference - #209
- [x] needless_borrows_for_generic_args - #223
- [x] needless_lifetimes
- #209
- #223
- [x] needless_maybe_sized - #209
- [x] new_without_default - #209
- [x] partialeq_to_none - #223
- [x] redundant_field_names - #223
- [x] redundant_static_lifetimes - #223
- [ ] result_large_err - #234
- [ ] result_unit_err - #235
- [x] search_is_some - #223
- [x] single_match - #223
- [x] suspicious_to_owned - #209
- [x] unnecessary_get_then_check - #223
- [x] unnecessary_mut_passed - #223
- [x] unused_unit - #223
- [x] useless_conversion
- #209
- #223
- [x] useless_vec - #223
## oxide-auth-async
- [ ] large_enum_variant - #234
- [x] mismatched_lifetime_syntaxes - #210
- [x] multiple_bound_locations - #224
- [x] needless_lifetimes - #224
- [ ] result_unit_err - #235
- [x] unnecessary_get_then_check - #224
- [x] useless_conversion - #210
- [x] useless_vec - #224
## oxide-auth-actix
- [x] mismatched_lifetime_syntaxes - #211
## oxide-auth-axum
- [x] mismatched_lifetime_syntaxes - #212
## oxide-auth-db
- [x] err_expect - #229
- [x] mismatched_lifetime_syntaxes - #213
- [x] needless_lifetimes - #213
- [x] needless_question_mark - #213
- [x] redundant_closure - #213
- [x] unused - #213
## oxide-auth-iron
- [x] from_over_into - #214
- [x] new_without_default - #214
## oxide-auth-poem
- [x] mismatched_lifetime_syntaxes - #215
## oxide-auth-rouille
- [x] mismatched_lifetime_syntaxes - #216
- [x] redundant_pattern_matching - #226
- [x] single_component_path_imports - #216
- [x] useless_conversion - #226
## oxide-auth-rocket
## actix-example
- [x] let_and_return - #225
- [x] useless_conversion - #225
- [x] type_complexity - #225
## db-example
- [x] useless_conversion - #227
- [x] type_complexity - #227

The issue lists specific clippy lint warnings across multiple crates (oxide-auth, oxide-auth-async, etc.) with linked pull requests showing resolved items. Start by running `cargo clippy` on the project to see current warnings. Focus on remaining unchecked items like 'large_enum_variant' or 'result_unit_err' in the tracking list. Use `cargo-semver-checks` to ensure fixes don't break semver. Look at merged PRs (e.g., #209, #223) for examples of the changes needed. Done when clippy runs cleanly for the targeted crates.

Rédigé par le modèle d'indexation à partir du texte de l'issue.