improve: overhaul config validation architecture -WoC

[aroradev1010]

Changes

• Replaced lazy getter pattern (get: () => value) — simpler and more direct
• Validation methods now return ValidationRule[] and aggregate into a single pass, removing the per-domain runRules() + validateProperty() indirection
• Added crossFieldIntRule helper to reduce repetition across cross-field comparisons
• Guarded all cluster sub-field validation behind cluster.enabled === true

Why

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Checkmarks

• The modified endpoints have been tested.
• Used the same indentation as the rest of the project.
• Still compatible with LavaLink clients.

Additional information

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