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/directory/src/otel.rs

@@ -23,6 +23,7 @@ pub fn provider(service: &'static str) -> anyhow::Result<Option<opentelemetry_sd
 /// one action sets off are one trace. None for a missing or malformed one. In the query where no
 /// header can be set: a position saved by sendBeacon, and the playlist Safari's player fetches
 /// itself (#192).
+#[must_use]
 pub fn remote_parent(headers: &axum::http::HeaderMap, query: Option<&str>) -> Option<opentelemetry::Context> {
     use opentelemetry::trace::{SpanContext, SpanId, TraceContextExt, TraceFlags, TraceId, TraceState};
     let v = match headers.get("traceparent") {
@@ -46,6 +47,7 @@ pub fn remote_parent(headers: &axum::http::HeaderMap, query: Option<&str>) -> Op
 
 /// The current span as a `traceparent`, for a request to another service, so its spans join
 /// this trace. None outside a traced span.
+#[must_use]
 pub fn traceparent() -> Option<String> {
     use opentelemetry::trace::TraceContextExt;
     use tracing_opentelemetry::OpenTelemetrySpanExt;