feat:Audit Logging for Sensitive Actions

[Skinny001]

• Add audit logging module (/lib/audit/) with types, logger, sanitizer, and middleware
• Log sensitive actions: login, logout, split updates, goals operations, bills creation
• Audit events include: timestamp, action, address, ip, resource, result
• Security: sanitize sensitive data (passwords, keys, tokens) from logs
• Support stdout (JSON) and database destinations
• Add AuditLog model to Prisma schema with indexes
• Integrate logging in 11 API routes (auth, goals, split, bills)
• Environment config: AUDIT_LOG_ENABLED, AUDIT_LOG_DESTINATION, AUDIT_RETENTION_DAYS

close #176