Implement cryptographically verifiable mint authorization log chain

[daree-dev]

This PR adds a dedicated, cryptographically verifiable audit trail for cNGN mint authorizations.

Key features:

• New append-only database table with hash chain verification
• Mint action logging integrated into onramp processor and transaction monitor
• Periodic chain integrity checks with critical security alerts
• Immutable WORM-style storage enforced by database triggers

Changes include:

• Migration: 20260401000000_mint_authorization_audit.sql
• New module: src/audit/mint_authorization.rs
• Updates to main.rs, onramp_processor.rs, transaction_monitor.rs

[drips-wave]

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Issue	Title
#215	Mint Authorization Audit Trail & Tamper-Evident Logging	Link to this issue

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