perf: SQLite cache_size増量、ノートフェッチバッチ拡大

- SQLite cache_size: Writer -4000→-16000 (64MB)、Reader -2000→-8000 (32MB)
  TL表示・検索のディスクI/O待ち削減
- ポーリング時のノートフェッチバッチ: 3→8
  リアクション更新の反映を高速化

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hitalin

src/db.rs

@@ -33,12 +33,12 @@ const PRAGMAS_WRITER: &str = "\
     PRAGMA foreign_keys=ON;\
     PRAGMA synchronous=NORMAL;\
     PRAGMA mmap_size=268435456;\
-    PRAGMA cache_size=-4000;\
+    PRAGMA cache_size=-16000;\
     PRAGMA temp_store=MEMORY;";
 
 const PRAGMAS_READER: &str = "\
     PRAGMA mmap_size=268435456;\
-    PRAGMA cache_size=-2000;\
+    PRAGMA cache_size=-8000;\
     PRAGMA temp_store=MEMORY;";
 
 /// SQLite database with separate reader/writer connections.

src/streaming.rs

@@ -1221,8 +1221,8 @@ async fn polling_loop(
                     .unwrap_or_default()
             };
 
-            // Fetch in batches of 3 to avoid overwhelming the server
-            for chunk in note_ids.chunks(3) {
+            // Fetch in batches of 8 to balance throughput vs server load
+            for chunk in note_ids.chunks(8) {
                 let futures: Vec<_> = chunk
                     .iter()
                     .map(|note_id| {