fix: escape LIKE wildcards in user-search ILIKE queries

A `%` or `_` in a search term silently acted as a LIKE wildcard rather than
matching literally (`50%` matched everything, `a_b` matched `axb`). Not
injection — terms are bound — but a search-correctness bug across every
user-facing ILIKE site.

- repo::escape_like: shared pub(crate) escaper (promoted from page_tag), unit-tested.
- Trigram-entangled sites (manga, tag, author) append a separate escaped param
  used only by the ILIKE branch; the trigram/similarity branches keep the raw term.
- Pattern-built sites (admin manga list, admin users, analysis coverage + history,
  crawler search incl. JSONB payload title) escape inside format!("%{}%", ..) and
  pair each ILIKE with ESCAPE '\'.

Tests: escape_like unit tests + integration tests on public manga search, author
autocomplete, and admin user search proving `_` matches literally.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/tests/api_admin_users.rs

@@ -147,6 +147,46 @@ async fn list_filters_by_substring_search(pool: PgPool) {
     assert_eq!(body["page"]["total"], 1);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn search_treats_like_wildcards_literally(pool: PgPool) {
+    let h = common::harness(pool.clone());
+    let (_admin_name, cookie, _) = seed_admin(&pool, &h.app).await;
+    // Two usernames differing only at one position (underscores are legal in
+    // usernames). `_` is a LIKE single-char wildcard: unescaped, `%a_b%` matches
+    // BOTH; escaped, only the literal "a_b" username. Admin user search has no
+    // trigram OR, so length/similarity don't matter here.
+    for username in ["axbfindme", "a_bfindme"] {
+        let resp = h
+            .app
+            .clone()
+            .oneshot(common::post_json(
+                "/api/v1/auth/register",
+                json!({ "username": username, "password": "hunter2hunter2" }),
+            ))
+            .await
+            .unwrap();
+        assert_eq!(resp.status(), StatusCode::CREATED);
+    }
+
+    let resp = h
+        .app
+        .oneshot(common::get_with_cookie(
+            "/api/v1/admin/users?search=a_b",
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    let items = body["items"].as_array().unwrap();
+    assert_eq!(
+        items.len(),
+        1,
+        "the `_` must match literally: only a_bfindme, not axbfindme"
+    );
+    assert_eq!(items[0]["username"], "a_bfindme");
+}
+
 // ---- self-protection -------------------------------------------------------
 
 #[sqlx::test(migrations = "./migrations")]