fix: add isolation level in transaction repository

[kushsharma]

Currently there were chances to have race conditions while writing to transaction repository. I have added a test to verify it doesn't happen. Database is using Serializable as the isolation level to avoid overlapping transactions.

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Name	Status	Preview	Comments	Updated (UTC)
frontier	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 4, 2024 3:43am

[coveralls]

Pull Request Test Coverage Report for Build 12152601272

Details

• 4 of 139 (2.88%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.002%) to 30.842%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/store/postgres/group_repository.go	0	15	0.0%
internal/store/postgres/organization_repository.go	0	19	0.0%
internal/store/postgres/billing_transactions_repository.go	0	101	0.0%
<!--	Total:	4	139	2.88%	-->

Files with Coverage Reduction	New Missed Lines	%
internal/store/postgres/billing_transactions_repository.go	2	3.06%
<!--	Total:	2		-->

Totals
Change from base Build 12112659884:	-0.002%
Covered Lines:	7630
Relevant Lines:	24739

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💛 - Coveralls