Add tooltip to ReputationBadge explaining how the score is calculated

[Kingsman-99]

Description

ReputationBadge shows a numerical reputation score but gives users no indication of how it is derived. Users who receive a lower score than expected have no context for what actions increase it. A tooltip with a brief breakdown (payments on time, dispute win rate, volume) adds transparency.

Acceptance Criteria

• Hovering the badge shows a tooltip with a 2-4 line breakdown of contributing factors
• Tooltip is accessible via keyboard focus
• The tooltip text is informational only and does not expose internal score weights
• Existing tests continue to pass

Context

• Target file: src/components/ReputationBadge.tsx