docs(backend): complete .env.example with all referenced environment variables

[Oyinkans0la12]

Closes #1101

Summary

Update backend/.env.example to include every environment variable referenced throughout the backend, making local setup more reliable and reducing configuration-related onboarding issues.

Problem

backend/.env.example is intended to be the canonical template for local development, but it does not currently include all environment variables consumed by the backend. As a result, new contributors can encounter runtime failures caused by missing configuration values that are not documented.

Solution

This PR audits all backend process.env.* references and synchronizes backend/.env.example so every required environment variable is represented with an appropriate placeholder value and brief documentation.

Implementation Details

Environment Variable Audit

Reviewed environment variable usage across backend configuration and supporting libraries, including:

• backend/src/config/**
• backend/src/lib/pg-pool.ts
• backend/src/lib/redis.ts
• backend/src/services/sorobanService.ts
• other backend modules referencing process.env.*

.env.example Updates

Updated:

• backend/.env.example

Changes include:

• adding missing environment variables,
• providing sensible placeholder/example values,
• organizing related configuration into logical sections,
• adding concise comments describing each variable's purpose.

Scope

This PR intentionally does not:

• introduce runtime environment validation,
• modify application defaults,
• rename configuration keys,
• change runtime behavior.

Testing

Validation performed by:

• auditing all process.env.* references,
• confirming each variable is documented in .env.example,
• verifying formatting consistency.

Behavioral Changes

None.

This PR improves configuration documentation only and does not affect application behavior.

Checklist

• Audited all backend process.env.* references.
• Added missing environment variables to .env.example.
• Added sensible placeholder values.
• Organized configuration into logical sections.
• Preserved existing runtime behavior.
• Ready for review.