feat: add IPFS CID metadata storage for creators

[Manuelshub]

Add IPFS CID Metadata Storage for Creators

Summary

Adds on-chain storage for a creator's IPFS CID, pointing to their profile, links, and subscription tier descriptions.

Changes

DataKey enum

• Added CreatorMetadata(Address) variant for per-creator metadata storage

New contract methods

• set_creator_metadata(creator, cid: Bytes) — stores the IPFS CID on-chain; requires creator auth so only the creator can update their own metadata
• get_creator_metadata(creator) -> Option<Bytes> — retrieves the stored CID, returns None if not yet set

Tests

• Added test_creator_metadata covering: initial None state, setting a CID, retrieving it, and updating to a new CID

Notes

• The CID is stored as raw Bytes to remain encoding-agnostic (works with CIDv0 and CIDv1)
• No migration needed — existing streams and storage keys are unaffected
• Also cleaned up pre-existing duplicate use statements and broken code fragments in lib.rs and test.rs

Closes #20

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