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/models/db_models.py

@@ -1,7 +1,7 @@
 import enum
 from datetime import datetime
 
-from sqlalchemy import Boolean, DateTime, Enum, ForeignKey, Integer, String, func
+from sqlalchemy import Boolean, DateTime, Enum, ForeignKey, Index, Integer, String, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
@@ -129,7 +129,7 @@ class FilterLog(Base):
     id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
     account_id: Mapped[int | None] = mapped_column(Integer, nullable=True)
     account_name: Mapped[str] = mapped_column(String(100), default="")
-    level: Mapped[LogLevel] = mapped_column(Enum(LogLevel), default=LogLevel.INFO)
+    level: Mapped[LogLevel] = mapped_column(Enum(LogLevel), default=LogLevel.INFO, index=True)
     message: Mapped[str] = mapped_column(String(1000))
     rule_name: Mapped[str | None] = mapped_column(String(200), nullable=True)
     action_type: Mapped[str | None] = mapped_column(String(50), nullable=True)
@@ -138,7 +138,11 @@ class FilterLog(Base):
     mail_from: Mapped[str | None] = mapped_column(String(255), nullable=True)
     folder: Mapped[str | None] = mapped_column(String(255), nullable=True)
     details: Mapped[str | None] = mapped_column(String(2000), nullable=True)
-    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now(), index=True)
+
+    __table_args__ = (
+        Index("ix_filter_logs_account_created", "account_id", "created_at"),
+    )
 
 
 class ProcessedMail(Base):