T3-021: Implement mint compressed NFT function

Track: 3 — Solana Phone / Web3
Section: Phase C: NFT Achievement Badges (72h)
Status: [ ] Todo
Repo: backend-api
Branch: feature/T3-021-mint-compressed-nft
Estimated Hours: 8h

Done When

Backend function: mint_badge(user_wallet, badge_type) -> mints compressed NFT to user's wallet via Bubblegum. Cost: <$0.01 per mint

Notes

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