fix: 검색 UX 개선 — Enter 키 기반 + 한국어 검색 ILIKE fallback

- 프론트: debounce 자동검색 제거 → Enter 키로만 검색 (한글 조합 문제 해결)
- 백엔드: trgm threshold 0.1로 낮춤 + ILIKE '%검색어%' fallback 추가
- hybrid 검색 score threshold 0.01 → 0.001로 낮춤

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/routes/documents/+page.svelte

@@ -63,27 +63,31 @@
     }
   }
 
-  function handleSearchInput() {
-    clearTimeout(debounceTimer);
-    debounceTimer = setTimeout(() => {
-      const params = new URLSearchParams($page.url.searchParams);
-      params.delete('page');
-      if (searchQuery.trim()) {
-        params.set('q', searchQuery.trim());
-      } else {
-        params.delete('q');
-      }
-      if (searchMode !== 'hybrid') {
-        params.set('mode', searchMode);
-      } else {
-        params.delete('mode');
-      }
-      for (const [key, val] of [...params.entries()]) {
-        if (!val) params.delete(key);
-      }
-      const qs = params.toString();
-      goto(`/documents${qs ? '?' + qs : ''}`, { noScroll: true });
-    }, 300);
+  function submitSearch() {
+    const params = new URLSearchParams($page.url.searchParams);
+    params.delete('page');
+    if (searchQuery.trim()) {
+      params.set('q', searchQuery.trim());
+    } else {
+      params.delete('q');
+    }
+    if (searchMode !== 'hybrid') {
+      params.set('mode', searchMode);
+    } else {
+      params.delete('mode');
+    }
+    for (const [key, val] of [...params.entries()]) {
+      if (!val) params.delete(key);
+    }
+    const qs = params.toString();
+    goto(`/documents${qs ? '?' + qs : ''}`, { noScroll: true });
+  }
+
+  function handleSearchKeydown(e) {
+    if (e.key === 'Enter') {
+      e.preventDefault();
+      submitSearch();
+    }
   }
 
   async function doSearch(q, mode) {
@@ -145,13 +149,13 @@
         data-search-input
         type="text"
         bind:value={searchQuery}
-        oninput={handleSearchInput}
-        placeholder="검색어 입력... (/ 키로 포커스)"
+        onkeydown={handleSearchKeydown}
+        placeholder="검색어 입력 후 Enter (/ 키로 포커스)"
         class="flex-1 px-3 py-1.5 bg-[var(--surface)] border border-[var(--border)] rounded-lg text-[var(--text)] text-sm focus:border-[var(--accent)] outline-none"
       />
       <select
         bind:value={searchMode}
-        onchange={() => { if (searchQuery) handleSearchInput(); }}
+        onchange={() => { if (searchQuery) submitSearch(); }}
         class="px-2 py-1.5 bg-[var(--surface)] border border-[var(--border)] rounded-lg text-[var(--text)] text-xs"
       >
         <option value="hybrid">하이브리드</option>