implement post /donations endpoint with validation and auth

[shreeyaadhikari]

ℹ️ Issue

Closes #245

📝 Description

Implements the POST /donations endpoint in donors.

Changes made:

1. Added POST /donations route in handler.ts that inserts a new row into branch.project_donations with donor_id, project_id, and amount
2. Added input validation that rejects missing fields, non-integer IDs, and non-positive amounts w/ a 400 response
3. Added a 409 response for duplicate donations
4. Added tests covering success case, validation error cases, and unauthenticated case

✔️ Verification

Added tests in \donors\test\donors.test.ts:

[mehanana]

looks great!