Add on-chain service rating system for subscription services

[Clawue884]

Summary

This PR introduces an on-chain service rating system for subscription services.

Subscribers can now rate services on a scale of 1 to 5, while the contract stores aggregate rating data on-chain.
This adds a lightweight reputation layer to merchant services without affecting the existing subscription workflow.

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Features Added

New Rating Model

Introduced a new ServiceRating struct to store:

• total_score
• total_raters

This enables calculation of average ratings on-chain.

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New Storage Keys

Added new persistent storage entries:

• ServiceRating(service_id)
  Stores aggregate rating data for each service.

• RatingGiven(subscriber, service_id)
  Prevents duplicate ratings by the same subscriber.

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New Contract Methods

rate_service(...)

Allows a subscriber to rate a service from 1 to 5.

Validation includes:

• service must exist
• score must be between 1 and 5
• duplicate ratings are rejected

On success:

• updates aggregate rating
• records subscriber rating status
• emits a rate event

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get_service_rating(...)

Returns the aggregate rating data for a service:

• total_score
• total_raters

Average rating can be derived as:

average = total_score / total_raters

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Error Handling

Added new contract errors:

• InvalidRating
• AlreadyRated

These ensure rating integrity and prevent invalid or duplicate submissions.

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Benefits

This feature adds:

• an on-chain reputation layer for services
• merchant quality visibility
• subscriber feedback transparency
• groundwork for future ranking or reputation systems

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Backward Compatibility

This PR is backward compatible:

• existing subscription flows are unchanged
• no breaking API changes
• rating functions are fully additive