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/auth.rs

@@ -46,7 +46,7 @@ const TOUCH_INTERVAL: Duration = Duration::hours(1);
 pub fn sha256_hex(text: &str) -> String {
     let digest = Sha256::digest(text.as_bytes());
     let mut hex = String::with_capacity(64);
-    for byte in digest.iter() {
+    for byte in &digest {
         let _ = write!(hex, "{byte:02x}");
     }
     hex