chore: error handling improvements, CI audit gate, legacy cleanup

[retardgerman]

Summary

• Better error messages in Discord interactions: Seerr failures in the request flow now differentiate between auth errors (401/403), server errors (5xx), and network errors (ECONNREFUSED/ETIMEDOUT) instead of showing a generic message
• CI: npm audit gate: Docker build now requires npm audit --audit-level=high to pass before building — blocks on new HIGH/CRITICAL CVEs automatically
• Removed legacy .env migration: parseEnvFile() and migrateEnvToConfig() were a one-time upgrade path from pre-config.json versions — removed; startup now logs a clear warning if a .env is found without a config.json
• Logging improvements: checkMediaStatus() now rethrows non-404 errors instead of silently returning { exists: false }; dailyPick.js channel fetch logs the full error object instead of just err.message
• JSDoc and comment fixes: Added @throws to checkMediaStatus, replaced a contradictory CRITICAL comment in sendRequest with an accurate explanation

AI-assisted documentation and code review; all logic changes written and verified manually.