Show plain-language explanations for each offer score

[imperator-clawdius]

The score is useful, but new users should understand why it changed.\n\nSuggested scope:\n- Show score factors in the UI: niche fit, pricing, margin, proof, fulfillment, lead source, clarity, and channels.\n- Add one plain-language sentence per factor.\n- Keep the existing scoring API compatible.\n\nVerification:\n- npm test\n- npm run smoke\n- Create a high-scoring and low-scoring offer and confirm explanations are readable