harden docker env loading and system fallback

[aaronkhall]

Summary

• make config loading Docker-safe by attempting .env load with godotenv.Load and falling back to process environment when the file is missing
• always bind configuration from process env after .env load, then normalize env-backed fields with strings.TrimSpace
• add explicit system key fallback from EXPLO_SYSTEM to MUSIC_SYSTEM_TYPE, normalize to lowercase, and log resolved system at startup for easier diagnosis
• add config tests covering env fallback precedence, whitespace trimming, and common-fix normalization behavior

Problem

In Docker runs, MUSIC_SYSTEM_TYPE / EXPLO_SYSTEM could resolve to an empty value, causing:
unknown system: ...

Test plan

• go test ./...