Add integration test for the buy endpoint recording a price snapshot after each confirmed transaction

[Chucks1093]

Summary

A price snapshot should be recorded in the database after every confirmed buy, capturing the new supply and price so the price history endpoint has accurate data. An integration test should confirm a snapshot is created for each buy and that the snapshot values match the post-buy contract state.

Scope

• Execute a buy of 5 keys and assert one price snapshot is created
• Assert the snapshot supply matches the post-buy supply
• Assert the snapshot price matches get_price at the new supply
• Execute a second buy and assert two total snapshots exist for that creator
• Assert no snapshot is created when a buy fails

Acceptance Criteria

• One snapshot created per confirmed buy
• Snapshot supply matches post-buy supply
• Snapshot price matches on-chain price at new supply
• No snapshot created on failed buy

ETA: 12 hours

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