docs: add migration documentation for DeletionRequest

Created comprehensive documentation for migration 1076 including:
- Overview and migration details
- Complete field descriptions
- How to apply and rollback
- Testing instructions
- Performance and security considerations

Co-authored-by: dawnsystem <42047891+dawnsystem@users.noreply.github.com>

docs/MIGRATION_1076_DELETION_REQUEST.md

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+# Migration 1076: DeletionRequest Model
+
+## Overview
+This migration adds the `DeletionRequest` model to track AI-initiated deletion requests that require explicit user approval.
+
+## Migration Details
+- **File**: `src/documents/migrations/1076_add_deletion_request.py`
+- **Dependencies**: Migration 1075 (add_performance_indexes)
+- **Generated**: Manually based on model definition
+- **Django Version**: 5.2+
+
+## What This Migration Does
+
+### Creates DeletionRequest Table
+The migration creates a new table `documents_deletionrequest` with the following fields:
+
+#### Core Fields
+- `id`: BigAutoField (Primary Key)
+- `created_at`: DateTimeField (auto_now_add=True)
+- `updated_at`: DateTimeField (auto_now=True)
+
+#### Request Information
+- `requested_by_ai`: BooleanField (default=True)
+- `ai_reason`: TextField - Detailed explanation from AI
+- `status`: CharField(max_length=20) with choices:
+  - `pending` (default)
+  - `approved`
+  - `rejected`
+  - `cancelled`
+  - `completed`
+
+#### Relationships
+- `user`: ForeignKey to User (CASCADE) - User who must approve
+- `reviewed_by`: ForeignKey to User (SET_NULL, nullable) - User who reviewed
+- `documents`: ManyToManyField to Document - Documents to be deleted
+
+#### Metadata
+- `impact_summary`: JSONField - Summary of deletion impact
+- `reviewed_at`: DateTimeField (nullable) - When reviewed
+- `review_comment`: TextField (blank) - User's review comment
+- `completed_at`: DateTimeField (nullable) - When completed
+- `completion_details`: JSONField - Execution details
+
+### Custom Indexes
+The migration creates two indexes for optimal query performance:
+
+1. **Composite Index**: `del_req_status_user_idx`
+   - Fields: `[status, user]`
+   - Purpose: Optimize queries filtering by status and user (e.g., "show me all pending requests for this user")
+
+2. **Single Index**: `del_req_created_idx`
+   - Fields: `[created_at]`
+   - Purpose: Optimize chronological queries and ordering
+
+## How to Apply This Migration
+
+### Development Environment
+
+```bash
+cd src
+python manage.py migrate documents 1076
+```
+
+### Production Environment
+
+1. **Backup your database first**:
+   ```bash
+   pg_dump paperless > backup_before_1076.sql
+   ```
+
+2. **Apply the migration**:
+   ```bash
+   python manage.py migrate documents 1076
+   ```
+
+3. **Verify the migration**:
+   ```bash
+   python manage.py showmigrations documents
+   ```
+
+## Rollback Instructions
+
+If you need to rollback this migration:
+
+```bash
+python manage.py migrate documents 1075
+```
+
+This will:
+- Drop the `documents_deletionrequest` table
+- Drop the ManyToMany through table
+- Remove the custom indexes
+
+## Backward Compatibility
+
+✅ **This migration is backward compatible**:
+- It only adds new tables and indexes
+- It does not modify existing tables
+- No data migration is required
+- Old code will continue to work (new model is optional)
+
+## Data Migration
+
+No data migration is required as this is a new model with no pre-existing data.
+
+## Testing
+
+### Verify Table Creation
+```sql
+-- Check table exists
+SELECT table_name 
+FROM information_schema.tables 
+WHERE table_name = 'documents_deletionrequest';
+
+-- Check columns
+\d documents_deletionrequest
+```
+
+### Verify Indexes
+```sql
+-- Check indexes exist
+SELECT indexname, indexdef 
+FROM pg_indexes 
+WHERE tablename = 'documents_deletionrequest';
+```
+
+### Test Model Operations
+```python
+from documents.models import DeletionRequest
+from django.contrib.auth.models import User
+
+# Create a test deletion request
+user = User.objects.first()
+dr = DeletionRequest.objects.create(
+    user=user,
+    ai_reason="Test deletion request",
+    status=DeletionRequest.STATUS_PENDING
+)
+
+# Verify it was created
+assert DeletionRequest.objects.filter(id=dr.id).exists()
+
+# Clean up
+dr.delete()
+```
+
+## Performance Impact
+
+- **Write Performance**: Minimal impact. Additional table with moderate write frequency expected.
+- **Read Performance**: Improved by custom indexes for common query patterns.
+- **Storage**: Approximately 1-2 KB per deletion request record.
+
+## Security Considerations
+
+- The migration implements proper foreign key constraints to ensure referential integrity
+- CASCADE delete on `user` field ensures cleanup when users are deleted
+- SET_NULL on `reviewed_by` preserves audit trail even if reviewer is deleted
+
+## Related Documentation
+
+- Model definition: `src/documents/models.py` (line 1586)
+- AI Scanner documentation: `AI_SCANNER_IMPLEMENTATION.md`
+- agents.md: Safety requirements section
+
+## Support
+
+If you encounter issues with this migration:
+1. Check Django version is 5.2+
+2. Verify database supports JSONField (PostgreSQL 9.4+)
+3. Check migration dependencies are satisfied
+4. Review Django logs for detailed error messages