[#632,#633,#634] Backend: Add standardized pagination envelope, secure audit redaction, and deterministic validation error schema/codes

[UzyKhs]

Summary

Implements three backend improvements:

• Standardized pagination envelope across list endpoints
• Secure audit redaction layer for sensitive log attributes
• Deterministic request validation error schema and stable error codes

Changes

• Added shared list envelope helpers and applied them to admin/list routes while preserving legacy compatibility fields
• Added recursive sensitive-attribute redaction utility and integrated it into structured logging and audit metadata capture
• Updated validation middleware to return deterministic sorted issues with stable machine-readable codes
• Added compatibility routing adjustments for legacy list and webhook verify routes

Verification

• TypeScript build compiles successfully
• Branch is mergeable with upstream main (no merge conflicts detected)

Closes #632
Closes #633
Closes #634

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