#523 - Centralize and type environment configuration

Parent: #523

## Objective
Create a single validated env/config surface to prevent late runtime failures and inconsistent module-level env access.

## In scope
- Introduce centralized env schema validation (required vars + type coercion)
- Export typed config consumed by runtime modules
- Migrate selected modules from direct `process.env` reads to typed config

## Acceptance criteria
- Startup fails fast with actionable errors when required env vars are missing/invalid
- Typed config object is the primary read path in touched modules
- Reduced ad-hoc env access in updated areas
- Tests cover invalid/missing env behavior

## Notes
Adopt incrementally to minimize merge conflict risk.