docs: comprehensively overhaul and sync documentation

[halcyon-past]

This PR performs a full audit and update of the project documentation to ensure all files accurately reflect the current state of the application.

Changes:

• frontend/README.md: Replaced Next.js boilerplate with Structurify-specific setup, testing, and Firebase deployment instructions.
• ENHANCEMENTS.md: Removed completed features (OAuth, History, IaC) and added new future roadmap goals (Stripe, PII Redaction, Team Workspaces).
• ARCHITECTURE.md: Updated the users collection schema to include tenant_id, workspaces, and subscription fields. Corrected the email threshold from 5MB to 1MB.
• README.md: Fixed 5MB to 1MB typos, resolved markdown formatting bugs, and added the Enterprise SSO feature.
• DEPLOYMENT.md: Added Firebase Auth/Identity Platform to the Secret Manager & Infrastructure dependencies.
• CONTRIBUTING.md: Standardized the Node.js requirement to >= 20 to match the rest of the project.

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