Add Coupon Center and Flashcard endpoints v2

[godie]

Implement Coupon and UserCoupon models, GET /v2/coupons/me, POST /v2/flashcards, and AI flashcard generator endpoint. Includes migrations, models, controllers, and tests.

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PR created automatically by Jules for task 9831978296190232588 started by @godie

[google-labs-jules]

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[godie]

the tests are broken need to fix it

[google-labs-jules]

the tests are broken need to fix it

I have fixed the tests. The issue was caused by enabling global fixtures which conflicted with some existing tests that manually clear the database. I've now limited the fixtures to only those necessary for the new tests, and all tests are passing now.