feat: add storage cleanup mechanism with soft delete and auto scheduler

- Add soft delete (deleted_at column) to preserve task records for statistics
- Implement cleanup service to delete old files while keeping DB records
- Add automatic cleanup scheduler (configurable interval, default 24h)
- Add admin endpoints: storage stats, cleanup trigger, scheduler status
- Update task service with admin views (include deleted/files_deleted)
- Add frontend storage management UI in admin dashboard
- Add i18n translations for storage management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/versions/f3d499f5d0cf_add_deleted_at_to_tasks.py

@@ -0,0 +1,34 @@
+"""add_deleted_at_to_tasks
+
+Revision ID: f3d499f5d0cf
+Revises: g2b3c4d5e6f7
+Create Date: 2025-12-14 12:17:25.176482
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f3d499f5d0cf'
+down_revision: Union[str, None] = 'g2b3c4d5e6f7'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Add deleted_at column for soft delete support."""
+    op.add_column(
+        'tool_ocr_tasks',
+        sa.Column('deleted_at', sa.DateTime(), nullable=True,
+                  comment='Soft delete timestamp - NULL means not deleted')
+    )
+    op.create_index('ix_tool_ocr_tasks_deleted_at', 'tool_ocr_tasks', ['deleted_at'])
+
+
+def downgrade() -> None:
+    """Remove deleted_at column."""
+    op.drop_index('ix_tool_ocr_tasks_deleted_at', table_name='tool_ocr_tasks')
+    op.drop_column('tool_ocr_tasks', 'deleted_at')