NFT achievement badges integration

[DevMuhdishaq]

Description
Bridge the gamification rewards/Badge system to the feature-flagged NFT suite so milestone badges can be minted as on-chain collectibles.

Direction

• Behind the experimental flag, on badge award in rewards, call nft_minting to mint a non-transferable (soulbound) achievement NFT to the user.
• Define achievement→NFT metadata mapping in nft_types.
• Guard so each achievement mints at most once per user.

Acceptance criteria

• Earning a badge mints exactly one corresponding NFT to the user.
• Re-earning the same badge does not mint a duplicate.
• Achievement NFTs are non-transferable (transfer attempts reject).
• Tests run under --features experimental and pass.
• No behavior change when the flag is disabled.

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[ustaxs]

Good day

I have read the requirements of this issue and i am confident that i can solve this

[grantfox-oss]

🦊 GrantFox — @ustaxs has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #218)
2. Your PR will be reviewed by the StelTade maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@ustaxs's PR #247 was approved and merged by @DevMuhdishaq.

🏆 @ustaxs: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (627 total points). Track your full progress on GrantFox.

👏 Great work, @ustaxs! Keep contributing to StelTade.