Server overhaul: clear clippy::pedantic wins

Taken from a -W clippy::pedantic scan, no allow added: map_or/map_or_else, method references instead of closures,
clone_from, write! instead of push_str(format!), borrowed Option<&str>, push_of without a needless Option, the
envelope's entity moved instead of cloned, the scheduler's plan() no longer a needless async fn (its doc now says
what it does on failure), sort_unstable, digit separators and inline format args in tests. Not taken: missing
# Errors/# Panics docs, must_use, doc backticks, assert!(x.is_empty()) in tests, too_many_lines.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/tests/sync_api/audit.rs

@@ -44,7 +44,7 @@ async fn generation_writes_recurring_audit_with_server_now_and_no_device() {
     assert_eq!((a.client_at.as_str(), a.server_at.as_str()), (NOW, NOW));
     assert_eq!(
         a.changes[3],
-        json!({"field": "amount_ore", "old": null, "new": 1250000})
+        json!({"field": "amount_ore", "old": null, "new": 1_250_000})
     );
     // The entry reaches the phones through pull.
     let page = pull(&app, &admin_token(), 0, None).await.json;