fix: tolerate losing the WAL-switch race (pragma needs exclusive lock)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/vg-index

@@ -36,7 +36,10 @@ def open_db():
     LIB.mkdir(parents=True, exist_ok=True)
     THUMBS.mkdir(parents=True, exist_ok=True)
     db = sqlite3.connect(DB, timeout=60)
-    db.execute("PRAGMA journal_mode=WAL")  # concurrent index jobs on one node
+    try:  # WAL is persistent once set; the switch itself needs an exclusive
+        db.execute("PRAGMA journal_mode=WAL")  # lock, so tolerate losing the race
+    except sqlite3.OperationalError:
+        pass
     db.executescript("""
     CREATE TABLE IF NOT EXISTS videos(
       id INTEGER PRIMARY KEY, path TEXT UNIQUE, mtime REAL, duration REAL,