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>

openspec/specs/task-management/spec.md

@@ -31,7 +31,7 @@ The OCR service SHALL generate both JSON and Markdown result files for completed
 - **AND** include enhanced structure from PP-StructureV3 or PyMuPDF
 
 ### Requirement: Task Detail View
-The frontend SHALL provide a dedicated page for viewing individual task details with processing track information and enhanced preview capabilities.
+The frontend SHALL provide a dedicated page for viewing individual task details with processing track information, enhanced preview capabilities, and file availability status.
 
 #### Scenario: Navigate to task detail page
 - **WHEN** user clicks "View Details" button on task in Task History page
@@ -61,6 +61,12 @@ The frontend SHALL provide a dedicated page for viewing individual task details
 - **AND** show bounding boxes overlay on preview
 - **AND** highlight different element types (headers, tables, lists) with distinct colors
 
+#### Scenario: Display file unavailable status
+- **WHEN** task has `file_deleted=True`
+- **THEN** page SHALL show file unavailable indicator
+- **AND** download buttons SHALL be disabled or hidden
+- **AND** page SHALL display explanation that files were cleaned up
+
 ### Requirement: Results Page V2 Migration
 The Results page SHALL use V2 task-based APIs instead of V1 batch APIs.
 
@@ -117,3 +123,77 @@ The system SHALL maintain detailed processing history for tasks including track
 - **AND** provide track selection statistics
 - **AND** include performance metrics for each processing attempt
 
+### Requirement: Soft Delete Tasks
+The system SHALL support soft deletion of tasks, marking them as deleted without removing database records to preserve usage statistics.
+
+#### Scenario: User soft deletes a task
+- **WHEN** user calls DELETE on `/api/v2/tasks/{task_id}`
+- **THEN** system SHALL set `deleted_at` timestamp on the task record
+- **AND** system SHALL NOT delete the actual files
+- **AND** system SHALL NOT remove the database record
+- **AND** subsequent user queries SHALL NOT return this task
+
+#### Scenario: Preserve statistics after soft delete
+- **WHEN** a task is soft deleted
+- **THEN** admin statistics endpoints SHALL continue to include this task's metrics
+- **AND** translation token counts SHALL remain in cumulative totals
+- **AND** processing time statistics SHALL remain accurate
+
+### Requirement: File Cleanup Scheduler
+The system SHALL automatically clean up old files while preserving database records for statistics tracking.
+
+#### Scenario: Scheduled file cleanup
+- **WHEN** cleanup scheduler runs (configurable interval, default daily)
+- **THEN** system SHALL identify tasks where files can be deleted
+- **AND** system SHALL retain newest N files per user (configurable, default 50)
+- **AND** system SHALL delete actual files from disk for older tasks
+- **AND** system SHALL set `file_deleted=True` on cleaned tasks
+- **AND** system SHALL NOT delete any database records
+
+#### Scenario: File retention per user
+- **WHEN** user has more than `max_files_per_user` tasks with files
+- **THEN** cleanup SHALL delete files for oldest tasks exceeding the limit
+- **AND** cleanup SHALL preserve the newest `max_files_per_user` task files
+- **AND** task ordering SHALL be by `created_at` descending
+
+#### Scenario: Manual cleanup trigger
+- **WHEN** admin calls POST `/api/v2/admin/cleanup/trigger`
+- **THEN** system SHALL immediately run the cleanup process
+- **AND** return summary of files deleted and space freed
+
+### Requirement: Admin Task Visibility
+Admin users SHALL have full visibility into all tasks including soft-deleted and file-cleaned tasks.
+
+#### Scenario: Admin lists all tasks
+- **WHEN** admin calls GET `/api/v2/admin/tasks`
+- **THEN** response SHALL include all tasks from all users
+- **AND** response SHALL include soft-deleted tasks
+- **AND** response SHALL include tasks with deleted files
+- **AND** each task SHALL indicate its deletion status
+
+#### Scenario: Filter admin task list
+- **WHEN** admin calls GET `/api/v2/admin/tasks` with filters
+- **THEN** `include_deleted=false` SHALL exclude soft-deleted tasks
+- **AND** `include_files_deleted=false` SHALL exclude file-cleaned tasks
+- **AND** `user_id={id}` SHALL filter to specific user's tasks
+
+#### Scenario: View storage usage statistics
+- **WHEN** admin calls GET `/api/v2/admin/storage/stats`
+- **THEN** response SHALL include total storage used
+- **AND** response SHALL include per-user storage breakdown
+- **AND** response SHALL include count of tasks with/without files
+
+### Requirement: User Task Isolation
+Regular users SHALL only see their own tasks and soft-deleted tasks SHALL be hidden from their view.
+
+#### Scenario: User lists own tasks
+- **WHEN** authenticated user calls GET `/api/v2/tasks`
+- **THEN** response SHALL only include tasks owned by that user
+- **AND** response SHALL NOT include soft-deleted tasks
+- **AND** response SHALL include tasks with deleted files (showing file unavailable status)
+
+#### Scenario: User cannot access other user's tasks
+- **WHEN** user attempts to access task owned by another user
+- **THEN** system SHALL return 404 Not Found
+- **AND** system SHALL NOT reveal that the task exists
+