fix: return 404 for missing rating event

[nanookclaw]

Fixes #571.

This updates the event rating handler so a request for a missing path event ID returns the existing AppError::NotFound response instead of falling through to database-error behavior.

The handler now verifies the path event_id after the ticket lookup and before the ticket/event ownership check. That keeps the existing ordering for invalid ratings and missing tickets, while preserving the 403 response when a real event is rated with a ticket from another event.

The aggregate update also uses fetch_optional and maps a no-row result to NotFound, so the transaction has the same behavior if the event row disappears before the UPDATE ... RETURNING * completes.

Verification:

• git diff --check

I also tried cargo fmt --check, cargo check --lib, and focused cargo test --lib ... commands. They currently fail before this change is checked because of unrelated existing compile/format issues in routes/mod.rs, EventSocialProofResponse Redis deserialization, and an existing SearchParams test initializer.

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