Fix github enterprise integration config silo error

[trillville]

Fixes InitializationError: Error getting github enterprise integration config caused by a silo boundary violation.

Previously, the DatabaseBackedIntegrationService directly accessed the control-plane Integration model from a REGION silo, leading to a SiloLimit.AvailabilityError and subsequent HTTP 400.

This PR modifies DatabaseBackedIntegrationService.get_integration to correctly route requests:

• In a REGION silo, it now uses the hybrid cloud integration_service for cross-silo communication.
• In CONTROL or MONOLITH silos, it continues with direct database access.

This ensures proper adherence to Sentry's silo architecture and resolves the InitializationError.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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