feat: implement internal balance tracking and accounting

[binayyub4211]

Closes #18

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Description

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[binayyub4211]

Title: feat: implement API key generation and hashing service

What was done:

Created POST /apikeys endpoint
Generates a 32-byte cryptographically secure random key prefixed with sp_live_
Stores only the SHA256 hash (key_hash) — never the plaintext key
Returns the plaintext key to the merchant only once upon creation
Closes #7

[MissBlue00]

@binayyub4211 CI/CD checks are failing, please resolve

[MissBlue00]

@binayyub4211 resolve the merge conflicts