refactor(config): standardize configuration naming

[germondai]

Summary

• standardize environment-variable namespaces and harden numeric parsing
• align Docker, Compose, API, capture, diagnostics, Redis, MITM, STT, and FFmpeg configuration
• document every rename and breaking migration without preparing a release

Verification

• bun run verify
• Docker Compose validation for all three Compose variants
• Docker build checks for both API Dockerfiles
• git diff --check