feat(config): add shaper environment variables

[TheArchitectit]

Part 1: Config (278 lines)

Add environment variables for RequestShaper feature.

Changes

• PLEXUS_REQUEST_SHAPER_ENABLED
• PLEXUS_REQUEST_SHAPER_QUEUE_DEPTH
• PLEXUS_REQUEST_SHAPER_QUEUE_TIMEOUT_MS
• PLEXUS_REQUEST_SHAPER_DEFAULT_RPM

Size: 278 lines ✅

[TheArchitectit]

Code Overview

This PR adds configuration infrastructure for the RequestShaper feature.

Key Changes:

1. packages/backend/src/config.ts - Added environment variable parsing
2. packages/backend/.env.example - Documented new variables

Architecture:

The configuration integrates with the existing PlexusConfig system, allowing runtime configuration of rate limiting parameters.

Testing:

• Config validation on startup
• Default values provided for backward compatibility

Review Notes:

• Check that default values are reasonable
• Verify backward compatibility with existing configs