feat: Implement discount programs uploader

[NewdlDewdl]

Overview

Implements the uploader for discount programs as requested in #122.

Changes

• Created uploader/discountsUploader.go following existing uploader patterns
• Updated main.go to handle -upload -discounts flag
• Updated README.md with upload documentation
• Enabled the discount pipeline in runners/weekly.sh
• Added unit tests for the discounts uploader

Implementation Details

• Uses UploadData[schema.DiscountProgram]
• Collection name: discounts (matches discounts.json filename convention)
• Handles file-not-found gracefully
• Replaces existing data (consistent with other standalone uploaders)

Testing

• Code builds successfully

Closes #122

[NewdlDewdl]

@mikehquan19 implemented the discounts uploader for #122.

Summary:

• Added uploader/discountsUploader.go (supports -upload -discounts)
• Updated docs and weekly runner
• Added unit tests for the discounts uploader

Would appreciate a quick review when you have time.

[mikehquan19]

Will do, thanks.

[NewdlDewdl]

@mikehquan19 follow-up: re-reviewed #122 and confirmed this PR matches the request (uploader implementation + docs + runner).

Local MongoDB integration sanity check:

• Started a temporary mongo:7 container
• Set MONGODB_URI=mongodb://localhost:27018
• Ran: ./api-tools -upload -discounts -i ./tmp/discounts_upload_test
• Verified combinedDB.discounts.countDocuments() == 2

So the uploader path works end-to-end against MongoDB.