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/src/openapi.rs

@@ -331,7 +331,8 @@ impl Modify for CommonRules {
 fn is_unauthenticated(operation: &Operation) -> bool {
     operation.security.as_ref().is_some_and(|requirements| {
         requirements.iter().all(|r| {
-            serde_json::to_value(r).is_ok_and(|v| v.as_object().is_some_and(|o| o.is_empty()))
+            serde_json::to_value(r)
+                .is_ok_and(|v| v.as_object().is_some_and(serde_json::Map::is_empty))
         })
     })
 }