Add shared event fixture factory for all event types

[Jayy4rl]

Context: Unit and integration tests across the codebase repeat the same boilerplate for constructing raw event objects. A fixture factory removes duplication.

Requirements:

• Create backend/src/test/fixtures/events.ts exporting builder functions: makeDepositEvent(overrides?), makeYieldDistributedEvent(overrides?), makeVaultCreatedEvent(overrides?), and equivalents for all parsed event types.
• Each builder returns a minimal valid raw event object with sensible defaults.

Acceptance criteria:

• All existing unit tests that use raw event objects can be refactored to use the fixture builders.
• Builders are importable from a single path.