test: tighten audit-flagged tests and add missing coverage

Tightens three tests whose names overstated what they checked:

- `login_succeeds_and_rotates_session` now asserts the login cookie
  differs from the registration cookie, and that the registration
  cookie is still valid after login (the documented contract).
- `storage::local::rejects_path_traversal` exercises three extra
  rejection paths the existing implementation already handled but the
  tests didn't probe: `a/./b`, the single-segment `.`, and the empty
  segment `a//b`.
- `create_and_use_bot_token` asserts that `token_hash` is *absent*
  from the response (`get(...).is_none()`), not just `is_null()`,
  which would have accepted an explicit `"token_hash": null` payload
  too.

Adds four coverage cases that the audit flagged as missing:

- `me_rejects_expired_session` — hand-craft a session row with
  `expires_at = now() - 1h`, hit `/auth/me` with the matching cookie,
  expect 401 + `unauthenticated`. Proves the extractor's
  `expires_at > now()` filter is wired.
- `concurrent_manga_bookmarks_serialised_by_unique_index` — spawn two
  POSTs in parallel for the same `(user, manga, chapter=null)`,
  assert one wins (201) and one collides (409) via the partial unique
  index from migration 0004.
- `bookmark_create_accepts_bearer_token` — mint a bot token and POST
  /bookmarks with `Authorization: Bearer`, asserting `CurrentUser`
  resolves identically to the cookie path on a write endpoint (not
  just `/auth/me`).
- Three new unit tests on `app::cors_layer` covering the allowlist
  (origin reflected, credentials true), a foreign origin (no
  allow-origin header emitted), and the same-origin default (empty
  allowlist emits no CORS headers at all).

`cors_layer` is `pub(crate)` now so the tests in `app::tests` can
reach it; the function itself is unchanged.

No version bump.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/api_bookmarks.rs

@@ -187,6 +187,90 @@ async fn user_a_cannot_delete_user_b_bookmark(pool: PgPool) {
     assert_eq!(resp.status(), StatusCode::NO_CONTENT);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn concurrent_manga_bookmarks_serialised_by_unique_index(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
+
+    let app_a = h.app.clone();
+    let cookie_a = cookie.clone();
+    let mid_a = manga_id.to_string();
+    let app_b = h.app.clone();
+    let cookie_b = cookie.clone();
+    let mid_b = manga_id.to_string();
+
+    let f1 = tokio::spawn(async move {
+        app_a
+            .oneshot(common::post_json_with_cookie(
+                "/api/v1/bookmarks",
+                json!({ "manga_id": mid_a }),
+                &cookie_a,
+            ))
+            .await
+            .unwrap()
+            .status()
+    });
+    let f2 = tokio::spawn(async move {
+        app_b
+            .oneshot(common::post_json_with_cookie(
+                "/api/v1/bookmarks",
+                json!({ "manga_id": mid_b }),
+                &cookie_b,
+            ))
+            .await
+            .unwrap()
+            .status()
+    });
+
+    let (s1, s2) = tokio::join!(f1, f2);
+    let statuses = [s1.unwrap(), s2.unwrap()];
+    assert!(
+        statuses.contains(&StatusCode::CREATED),
+        "expected one winner with 201, got {statuses:?}"
+    );
+    assert!(
+        statuses.contains(&StatusCode::CONFLICT),
+        "expected one loser with 409 (the partial unique index), got {statuses:?}"
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn bookmark_create_accepts_bearer_token(pool: PgPool) {
+    // Bot scripts use Authorization: Bearer; cover that path on a
+    // *write* endpoint to make sure CurrentUser resolves identically
+    // whether the credential is a cookie or a bearer.
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::post_json_with_cookie(
+            "/api/v1/auth/tokens",
+            json!({ "name": "ci-bot" }),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    let bearer = common::body_json(resp).await["bearer"]
+        .as_str()
+        .unwrap()
+        .to_string();
+
+    let resp = h
+        .app
+        .oneshot(common::post_json_with_bearer(
+            "/api/v1/bookmarks",
+            json!({ "manga_id": manga_id.to_string() }),
+            &bearer,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::CREATED);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn delete_unknown_bookmark_is_404(pool: PgPool) {
     let h = common::harness(pool);