Further improve average remediation time queries

[Hubtrick-Git]

• We do not need to run the same query 8 times to check different risk/cvss ranges, rather we can just use conditional aggregation to do this for us.
• This improves the speed drastically since we only need to do 1/8 of the queries and we can also cut down a lot of duplicate code in different parts
• Also refactored parts relying on these queries

[Copilot]

Pull request overview

This PR refactors average remediation time (fixing time) calculations to use a single SQL query with conditional aggregation, reducing repeated queries and consolidating returned metrics for both risk score and CVSS bands.

Changes:

• Replace per-severity average-fixing-time queries with one aggregated query returning multiple buckets at once.
• Introduce dtos.RemediationTimeAverages to carry risk/CVSS bucket averages.
• Refactor controllers/services to remove now-redundant severity-based average-fixing-time methods and wire the new return type through.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
shared/common_interfaces.go	Updates repository/service interfaces to reflect the new aggregated return type and removes artifact-scoped CVSS method.
services/statistics_service.go	Removes artifact-scoped severity/CVSS average-fixing-time methods (no longer needed with aggregated query).
dtos/statistics_dto.go	Adds RemediationTimeAverages DTO for bucketed averages.
database/repositories/statistics_repository.go	Implements conditional-aggregation SQL returning all risk/CVSS averages in one call.
controllers/statistics_controller.go	Changes the average-fixing-time endpoint to return the aggregated DTO and removes severity handling.
controllers/artifact_controller.go	Uses aggregated averages in PDF report generation and adds a parsing helper.

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