Follow the Rust style guide's smaller rules (#353)

Measured against /src/styleguide/rustguide.md:
- Each use statement imports from one module, and free functions are
  called through theirs (db::step, db::now, jsonwebtoken::decode,
  crate::feed::redact, unix::signal) rather than imported bare.
- unsafe_code is forbidden workspace-wide. The one unsafe was a test's
  set_var, which is unsound while other tests' threads read the
  environment; the test now reads PATH, which every run has.
- Locks say why they may panic: expect("a panic elsewhere poisoned this
  lock") in place of 25 unwraps, and unique_slug's endless search says
  it cannot end.
- No combinator chains in for headers: each is a named variable first.
- #[must_use] on the 27 functions clippy's must_use_candidate names.
- A literal bool passed where its meaning is not plain gets its name
  (/* keep_open */ true, /* piece */ false, ...).

No behaviour changes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -7,6 +7,10 @@ resolver = "3"
 [workspace.package]
 edition = "2024"
 
+# None of ours needs unsafe, and the style guide asks for none where safe code will do.
+[workspace.lints.rust]
+unsafe_code = "forbid"
+
 [workspace.dependencies]
 ipx-feed = { path = "crates/feed" }
 ipx-directory = { path = "crates/directory" }