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!

[nshoreibah-kvyo]

Couple easy comments but looks good!

[nshoreibah-kvyo]

Can we add this GET /donations back?

[shreeyaadhikari]

The README hasn't changed on my local machine, I'm not quite sure how to change this.
Tried to change it on GitHub but wouldn't let me.

[nourshoreibah]

You should be able to fix it by pulling main then running git checkout main -- apps/backend/lambdas/donors/README.md to use the version from main, making the change to add the new API method, then committing!

[shreeyaadhikari]

I did that and it worked, but once the checks started running the GET/donations changed to POST/donations

[nourshoreibah]

Oh weird it must be a bug with CI then, sorry feel free to merge

[nourshoreibah]

I just realized the bot only sees request changes so requesting changes