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/pull.rs

@@ -28,7 +28,7 @@ fn versions(page: &Value) -> Vec<i64> {
                 .map(|r| r["server_version"].as_i64().unwrap())
         })
         .collect();
-    all.sort();
+    all.sort_unstable();
     all
 }
 
@@ -99,7 +99,7 @@ async fn the_first_page_has_everything_in_version_order_with_the_epoch_and_serve
             .map(|r| r["server_version"].as_i64().unwrap())
             .collect();
         let mut sorted = v.clone();
-        sorted.sort();
+        sorted.sort_unstable();
         assert_eq!(v, sorted, "{a} sorted by server_version");
     }
     // Children come with the parent.