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>

crates/feed/src/guard.rs

@@ -28,6 +28,7 @@ impl Allowed {
         list.iter().map(|s| parse_net(s)).collect::<Result<Vec<_>>>().map(|v| Self(Arc::new(v)))
     }
 
+    #[must_use]
     pub fn permits(&self, ip: IpAddr) -> bool {
         let ip = canonical(ip);
         !is_internal(ip) || self.0.iter().any(|&(net, bits)| contains(net, bits, ip))
@@ -73,6 +74,7 @@ fn contains(net: IpAddr, bits: u8, ip: IpAddr) -> bool {
 /// unstable.
 // shortcut: 6to4 (2002::/16) and Teredo addresses that wrap an internal IPv4 one pass; check the
 // embedded address if an IPv6 route to them ever exists here.
+#[must_use]
 pub fn is_internal(ip: IpAddr) -> bool {
     match canonical(ip) {
         IpAddr::V4(a) => {
@@ -107,6 +109,7 @@ pub fn is_internal(ip: IpAddr) -> bool {
 pub struct Guard(Allowed);
 
 impl Guard {
+    #[must_use]
     pub fn new(allowed: Allowed) -> Self {
         Self(allowed)
     }