feat: implement soft delete functionality for snippets

[devcarole]

Closes #70

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• Add isDeleted, deletedAt, deletedBy columns to snippets table
• Create activity_logs table for audit trail
• Implement soft delete in repository and service layers
• Add trash endpoint (GET /api/snippets/trash)
• Add restore endpoint (POST /api/snippets/[id]/restore)
• Add activity history endpoint (GET /api/snippets/[id]/activity)
• Create ActivityLogger utility for audit logging
• Update query filtering to exclude soft-deleted snippets
• Add ownership verification for restore operations
• Create 7 performance indexes for optimized queries
• Add comprehensive documentation (10 files)
• Add database migration script

Acceptance Criteria Met:
✅ isDeleted flag in schema
✅ Deleted snippets excluded from normal queries
✅ Trash/recovery section with list endpoint
✅ Restore functionality implemented
✅ Activity logging for delete/restore actions

Closes: Add Soft Delete Functionality for Snippets

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[devcarole]

Dear maintainer, check and merge PR!

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