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-import/tests/import.rs

@@ -549,7 +549,7 @@ async fn header_failures_write_nothing() {
     for file in ["bad_header.csv", "wrong_member.csv"] {
         match db.import(&db.options(file, "Shared")).await {
             Err(e @ ImportError::Header(_)) => {
-                assert_eq!(e.exit_code(), kvitt_import::EXIT_INVALID)
+                assert_eq!(e.exit_code(), kvitt_import::EXIT_INVALID);
             }
             other => panic!("{file}: {other:?}"),
         }