[jrodal98]

Applies the simple-rust skill (now saved as a project skill) across the codebase. Verified end state: cargo fmt --check clean, cargo clippy --all-targets -- -D warnings clean, full cargo test suite green. Independent of #61 (disjoint files; merge in any order).

Formatting: the tree was never rustfmt-formatted (360 diff hunks). This PR runs cargo fmt over everything; most of the diff is that mechanical pass.

Semantic changes:

• No #[allow] remains in src. AdminUser dead_code allow removed outright — an #[expect] conversion flagged it as unfulfilled, i.e. the suppression was already stale (exactly what expect is for). too_many_arguments on db::create_bookmark removed properly: new db::NewBookmark params struct; all 4 production and 16 test call sites converted. unused_mut on the session-cookie builder removed by shadowing the builder under the release-only cfg instead of mutating it. while_let_on_iterator in the template parsers quoted-value reader converted to a for loop over chars.by_ref().
• Clippy result_large_err x3 in the JSON API helpers: full Response (128+ bytes) was the Err of internal Results. New small ApiError payload struct (status + message) with IntoResponse; the {"error": ...} wire shape is byte-identical.
• AppError now derives thiserror (Display strings unchanged); manual Display/Error impls deleted. main() returns anyhow::Result instead of Box. Four production .unwrap() calls became .expect(...) with reasons (governor config, response builder, two export header parses); the remaining non-test expects already carried reasons or are fail-fast startup. e2e_test single_match -> if let.
• Cargo.toml: dependencies alphabetized (dev-deps too), async-trait removed (zero usages anywhere — machete-style check by grep), thiserror = 2 added, rust-version = 1.81 declared (the floor for #[expect] with reason). Cargo.lock stays gitignored per existing repo policy.

Deliberately unchanged (local convention wins over the guide): mod.rs module layout throughout; update_bookmark keeps 7 positional args (at, not over, the clippy threshold); grouped db/service layering as-is. Suggestion for later (needs workflow-edit permission the Muse app token lacks): a CI job running fmt --check + clippy -D warnings + test so the gates stay green.