Improve performance and harden reliability across AB testing, auth, idempotency, and anomaly detection

[AbdulSnk]

Summary

This PR improves performance, resilience, and configurability across several core services by addressing four related issues.

Changes

AB Testing

• Removed N+1 database writes by batching variant persistence.
• Wrapped variant writes in a transaction to ensure atomic updates.
• Audited and updated statistical analysis where applicable.
• Added tests covering bulk writes and transaction rollback.

Idempotency

• Hardened JSON.parse() with error handling.
• Treat corrupt or malformed records as cache misses.
• Validated parsed payloads before use.
• Added logging and self-healing behaviour for invalid records.
• Added tests for valid, malformed, and corrupt payloads.

Social Authentication

• Protected OAuth provider payload parsing with try/catch.
• Validated provider profile structure before use.
• Enforced verified-email checks before account creation or linking.
• Returned UnauthorizedException for malformed provider responses.
• Added tests covering malformed payloads, invalid schemas, and unverified emails.

Anomaly Detection

• Moved detection thresholds into configuration with sensible defaults.
• Guarded against NaN and invalid inputs.
• Improved detection stability using safeguards for sparse/invalid data.
• Added tests for configurable thresholds, boundary conditions, and invalid inputs.

Testing

• Added and updated unit tests covering all new functionality and edge cases.
• Preserved existing behaviour while improving performance, resilience, and configurability.

Notes

The repository contains pre-existing unrelated TypeScript build errors on the base branch. This PR is limited to the scope of the four assigned issues and does not modify unrelated code.

Closes: #1127
Closes: #1129
Closes: #1130
Closes: #1140

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