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/main.rs

@@ -2,7 +2,8 @@
 
 use anyhow::{Context, Result};
 use clap::{Parser, Subcommand};
-use ipx_directory::{Directory, config, store::Store};
+use ipx_directory::store::Store;
+use ipx_directory::{Directory, config};
 use tracing_subscriber::prelude::*;
 
 #[derive(Parser)]
@@ -101,7 +102,7 @@ async fn serve(path: &std::path::Path) -> Result<()> {
         .ok()
         .filter(|u| !u.trim().is_empty())
         .with_context(|| format!("{} is not set: the directory needs a database URL", cfg.database_url_env))?;
-    let store = Store::open(&url, true).await.context("opening the database")?;
+    let store = Store::open(&url, /* keep_open */ true).await.context("opening the database")?;
     let dir = Directory::new(store, vec![], ipx_feed::guard::Allowed::parse(&cfg.fetch_internal)?)?;
     let dropped = dir.set_tokens(tokens).await?;