docs: add code audit, implementation guide, userflow & testing docs

[rahuliitk]

Summary

• codeaudit.md: Comprehensive code audit report covering 278 findings across security (16 critical/high), frontend (30), backend (14), database (3), ML (4), infrastructure (20+), testing gaps, UI/UX issues (25), requirements gap analysis (307 features mapped), and desired enhancements (51)
• auditissuesimplementation.md: Detailed step-by-step remediation guide for all 278 findings, organized into priority phases (Weeks 1-10), with exact file paths, current vulnerable code, and fixed replacement code
• userflow.md: User flow documentation for all 44 features
• testingguide.md: Testing guide for the platform
• features2402.md: Updated feature status report

Test plan

• Verify markdown renders correctly on GitHub
• All file paths referenced in audit match actual codebase structure
• Finding IDs are consistent between codeaudit.md and auditissuesimplementation.md

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