Server overhaul: one test harness in tests/common for every test binary

core_api/common.rs becomes tests/common/mod.rs and sync_api/m4.rs tests/common/sync_fixtures.rs; each binary
includes them through #[path] (the binaries of SPEC §14.7 stay as they are). TempDir's Default moves into the
harness instead of three copies; the skeleton helpers take ids, tokens and TempDir from it. Milestone wording in
test docs and the server README replaced by what the code does; README seams and test layout updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/tests/push_fcm/fcm.rs

@@ -16,8 +16,8 @@ use serde_json::{Value, json};
 use uuid::Uuid;
 
 use crate::common::*;
-use crate::m4;
 use crate::stub::*;
+use crate::sync_fixtures;
 
 const EXPENSE: &str = "01929c5e-1111-7aaa-8bbb-000000000001";
 const FAST: [u64; 3] = [10, 20, 40];
@@ -31,7 +31,7 @@ struct Setup {
 
 impl Setup {
     async fn new() -> Self {
-        let app = m4::app().await;
+        let app = sync_fixtures::app().await;
         for (device, tok) in [
             (ADMIN_DEVICE_ID, "tok-admin"),
             (MEMBER_DEVICE_ID, "tok-member"),
@@ -445,12 +445,12 @@ async fn sync_push_reaches_fcm_through_the_queue_and_the_sender_task() {
             delays: vec![Duration::from_millis(10)],
         },
     );
-    m4::with_push(&mut s.app, queue);
-    let id = m4::new_id();
-    m4::push_one(
+    sync_fixtures::with_push(&mut s.app, queue);
+    let id = sync_fixtures::new_id();
+    sync_fixtures::push_one(
         &s.app,
         &member_token(),
-        m4::exp(m4::expense(&id, "2026-10-17T11:00:00.000Z")),
+        sync_fixtures::exp(sync_fixtures::expense(&id, "2026-10-17T11:00:00.000Z")),
     )
     .await;
     let deadline = Instant::now() + Duration::from_secs(5);