DonationsRepository: admin-focused queries and helpers

[thaninbew]

Summary:

Provide a dedicated repository wrapper for admin use-cases --- paginated lists, filters, aggregate queries, and dedicated search patterns. This makes complex SQL/TypeORM queries testable and keeps service small.

Files to create:

• apps/backend/src/donations/donations.repository.ts (new)

Suggested repository methods (at least these 5):

• findPaginated(page: number, perPage: number, filters?: Record<string, any>): Promise<{ rows: Donation[]; total: number }>
• searchByDonorNameOrEmail(q: string, limit?: number): Promise<Donation[]>
• getTotalsByDateRange(start: Date, end: Date): Promise<{ total: number; count: number }>
• findRecentPublic(limit: number): Promise<PublicDonationDto[]>
• deleteById(id: number): Promise<void> (admin-only destructive operation)

Acceptance criteria:

• Methods implemented using TypeORM QueryBuilder or repository methods and unit-tested (mock DB or run against a test DB).
• Service file DonationsService (business logic and tests) (Part 2) #9 will use this repository for admin endpoints or complex queries.

Testing steps:

1. Unit test repository methods with a local test database or with a mocked TypeORM QueryBuilder.