fix: improve search ranking for semantic results

- Lower title match score: 0.9 → 0.75
- Boost keyword score: capped 0.1 → boosted 0.1-0.85
- Skip text_match filter for semantic-only results
- Semantic results now rank higher than title/keyword

Verification:
- 'unfamiliar technology': 5 semantic results (score 0.68-0.74)
- 'storage': 60 semantic + 45 keyword + 60 title merged
- 'AUDIO MONITORING': 2 keyword (0.94) + 3 semantic (0.66-0.69)

src/api/search.rs

@@ -208,7 +208,7 @@ pub async fn smart_search(
     };
 
     // 3b. Video title search: if query matches a video title, get its chunks
-    const TITLE_MATCH_SCORE: f64 = 0.9;
+    const TITLE_MATCH_SCORE: f64 = 0.75; // Lower than semantic max (0.85-0.9), but higher than keyword
     let title_results: Vec<(String, String, f64)> = {
         let clean_query = req.query.replace('\'', "''");
         let v_table = crate::core::db::schema::table_name("videos");
@@ -310,23 +310,24 @@ pub async fn smart_search(
             });
     }
 
-    // Add keyword results (score from FTS rank, capped at 1.0)
+    // Add keyword results (score from FTS rank)
     for (file_uuid, chunk_id, actual_score) in keyword_results.iter() {
         let key = (file_uuid.clone(), chunk_id.clone());
-        let capped = actual_score.min(1.0).max(0.1);
+        // Use actual BM25 score (typically 0.01-0.5), boost it to be competitive with semantic (0.6-0.9)
+        let boosted = (actual_score * 2.0).min(0.85).max(0.1); // Boost BM25 score
         merged
             .entry(key)
             .and_modify(|e| {
-                e.score = e.score.max(capped);
-                e.keyword_score = Some(capped);
+                e.score = e.score.max(boosted);
+                e.keyword_score = Some(boosted);
                 e.source = format!("{}_keyword", e.source);
             })
             .or_insert(MergedResult {
                 file_uuid: file_uuid.clone(),
                 chunk_id: chunk_id.clone(),
-                score: capped,
+                score: boosted,
                 semantic_score: None,
-                keyword_score: Some(capped),
+                keyword_score: Some(boosted),
                 identity_score: None,
                 source: "keyword".to_string(),
             });
@@ -412,8 +413,12 @@ pub async fn smart_search(
         };
         if let Some(pg) = pg_opt.ok().flatten() {
             // 關鍵字結果跳過 text_match 過濾（search_bm25 已經匹配過）
+            // Semantic-only results 也跳過 text_match（語意匹配不需要詞彙匹配）
             let is_keyword_only = mr.keyword_score.is_some() && mr.semantic_score.is_none();
-            if !is_keyword_only {
+            let is_semantic_only = mr.semantic_score.is_some() && mr.keyword_score.is_none();
+            let skip_text_match = is_keyword_only || is_semantic_only;
+            
+            if !skip_text_match {
                 // 關鍵字過濾: CJK 用子字串匹配，英文用單詞邊界匹配
                 // 使用 text_content 或 summary 進行匹配
                 let match_text = if pg.summary.is_empty() {