[HIGH] Create comprehensive .env.example files for all services

[kilodesodiq-arch]

Engineering Gap

No .env.example files found for backend, frontend, mobile, or onchain services. Environment variables documented only in code comments and service READMEs. 50+ env vars used across services with no centralized configuration reference.

Codebase Evidence

• No .env.example in app/backend/, app/frontend/, app/mobile/, app/onchain/
• app/backend/src/main.ts lines 21-26: searches for .env in 3 locations at startup
• app/frontend/src/lib/env.ts: 10+ NEXT_PUBLIC_* vars without example file
• app/mobile/src/config/index.ts: 8 EXPO_PUBLIC_* vars without example
• app/ai-service/config.py: Settings class with env vars but no example file

Risk Profile

Current Risk

Onboarding friction — developers guess at required env vars. Production misconfiguration from missing documentation.

Business Impact

Slower developer onboarding. Production incidents from missing env vars. Non-reproducible builds.

Remediation Strategy

Create .env.example in each service with all required/supported vars, sensible defaults, and descriptions. Reference in each README.

Success Conditions

• app/backend/.env.example with all vars documented
• app/frontend/.env.example with NEXT_PUBLIC_* documented
• app/mobile/.env.example with EXPO_PUBLIC_* documented
• app/onchain/.env.example with contract/network config

Change Surface

New files: 4 .env.example files across services

Security Review

No secrets in example files — use placeholder values only. No security impact.

Completion Checklist

• All 4 .env.example files created
• READMEs updated
• Ready for merge