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>

alembic/versions/3bccad0c6646_add_filter_logs_indexes.py

@@ -0,0 +1,48 @@
+"""add indexes on filter_logs
+
+Revision ID: 3bccad0c6646
+Revises: c14c86cbc9c0
+Create Date: 2026-05-18 00:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+
+revision: str = "3bccad0c6646"
+down_revision: Union[str, Sequence[str], None] = "c14c86cbc9c0"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Indizes für schnelle Log-Abfragen bei großen Tabellen.
+
+    Beschleunigt: ORDER BY created_at DESC, Filter nach account_id/level,
+    und die kombinierte (account_id + ORDER BY created_at)-Abfrage.
+    """
+    op.create_index(
+        "ix_filter_logs_created_at",
+        "filter_logs",
+        ["created_at"],
+        unique=False,
+    )
+    op.create_index(
+        "ix_filter_logs_level",
+        "filter_logs",
+        ["level"],
+        unique=False,
+    )
+    op.create_index(
+        "ix_filter_logs_account_created",
+        "filter_logs",
+        ["account_id", "created_at"],
+        unique=False,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_filter_logs_account_created", table_name="filter_logs")
+    op.drop_index("ix_filter_logs_level", table_name="filter_logs")
+    op.drop_index("ix_filter_logs_created_at", table_name="filter_logs")