safe move + sqlite WAL + log indexes + backup with logs

- fix: imap folder names with spaces (RFC3501 quoting in move/create/select)
- fix: move only deletes source after COPY + Message-ID verification in target
- fix: backup endpoint hung on sqlite write locks — enable WAL + busy_timeout
- perf: indexes on filter_logs(created_at, level, account_id+created_at) for
  fast log queries on millions of rows
- feat: optional "logs mit sichern" checkbox in backup export, restore on import
- UI: backup download uses fetch+blob with error display instead of location.href

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/database.py

@@ -1,9 +1,24 @@
-from sqlalchemy import create_engine
+from sqlalchemy import create_engine, event
 from sqlalchemy.orm import DeclarativeBase, sessionmaker
 
 from app.config import settings
 
 engine = create_engine(settings.database_url, connect_args={"check_same_thread": False})
+
+
+# SQLite: WAL-Modus, damit Reader (z.B. Backup-Export) nicht von gleichzeitigen
+# Writern (Scheduler/Log) blockiert werden. busy_timeout sorgt dafür, dass kurze
+# Lock-Konflikte automatisch retryen statt sofort zu failen.
+if settings.database_url.startswith("sqlite"):
+    @event.listens_for(engine, "connect")
+    def _set_sqlite_pragmas(dbapi_connection, _):
+        cursor = dbapi_connection.cursor()
+        cursor.execute("PRAGMA journal_mode=WAL")
+        cursor.execute("PRAGMA busy_timeout=10000")
+        cursor.execute("PRAGMA synchronous=NORMAL")
+        cursor.close()
+
+
 SessionLocal = sessionmaker(bind=engine)