fix: source_post_id INTEGER → BIGINT (Synology Chat post_id 범위 초과)

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

init/migrate-v6.sql

@@ -14,7 +14,7 @@ CREATE TABLE IF NOT EXISTS retrospect.entries (
     confidence      REAL DEFAULT 0.0,
     source          VARCHAR(20) DEFAULT 'chat',
     reviewed        BOOLEAN DEFAULT FALSE,
-    source_post_id  INTEGER,
+    source_post_id  BIGINT,
     username        VARCHAR(100)
 );