Add Full-Stack Layers Perspective

[mavishay]

Summary

Adds a complementary 13-layer perspective to the existing 12 factors, covering backend, infrastructure, and operations concerns from a fullstack developer's view.

Changes

• Housekeeping: Renamed assests/ → assets/ (typo fix), added VISION.md and CONTRIBUTING.md
• README: Added Layers table of contents and intro reference
• CODEOWNERS: Added layers/* @mavishay
• New layers/ directory (15 files):
  • 00-Intro.md — overview of the 13-layer model
  • 01-Layer-1-Frontend.md through 13-Layer-13-Availability-and-Recovery.md — one article per layer
  • LAYERS-MATRIX.md — bidirectional cross-reference to existing 12 factors
• Images: 14 cover images for each layer article

Key Features

• Each article follows the same format as existing factors (TL;DR, Why It Matters, Key Considerations, Implementation Patterns, Common Pitfalls, Cross-References, Case Study, Conclusion)
• Heavy cross-linking to relevant existing factors
• Consistent style and voice with the existing articles
• ~9,600 lines of new content across 37 files

Layers Covered

1. Frontend
2. APIs & Backend Logic
3. Database & Storage
4. Auth & Permissions
5. Hosting & Deployment
6. Cloud & Compute
7. CI/CD & Version Control
8. Security & RLS
9. Rate Limiting
10. Caching & CDN
11. Load Balancing & Scaling
12. Error Tracking & Logs
13. Availability & Recovery

Test Plan

• All cross-reference links resolve to existing article files
• All image references resolve to existing images
• All articles have required sections, ≥300 lines, no placeholders
• Working tree is clean, commits follow conventional format