improve: add range, enum, and cross-field config validation

[aroradev1010]

Changes

This PR strengthens configuration validation at startup by adding:

• Range validation for numeric config values (ports, intervals, thresholds)
• Enum validation for fixed-value options (audio quality, route planner strategy)
• Cross-field validation to ensure related values are coherent (e.g. rate-limit hierarchy, cluster worker limits)
• Clear, actionable error messages when invalid configurations are detected

Why

Previously, many configurations were only type-checked or relied on runtime failures.
This allowed logically invalid but syntactically valid configurations to pass silently and fail later.

These changes:

• Fail fast during startup
• Prevent subtle runtime bugs

Scope

• No API changes
• No runtime behavior changes when configuration is valid
• Validation only runs during startup

Checkmarks

• Tested locally with valid and invalid configurations