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/openapi_contract.rs

@@ -206,7 +206,7 @@ fn canon(value: &Value, names: bool) -> Value {
                             Some(Value::Array(inner))
                                 if alternative.as_object().is_some_and(|o| o.len() == 1) =>
                             {
-                                flat.extend(inner.iter().cloned())
+                                flat.extend(inner.iter().cloned());
                             }
                             _ => flat.push(alternative),
                         }
@@ -383,7 +383,7 @@ fn generated_document_equals_contract() {
             paths
                 .values()
                 .filter_map(Value::as_object)
-                .map(|m| m.len())
+                .map(serde_json::Map::len)
                 .sum::<usize>()
         })
         .unwrap_or(0);