feat: add minimum price

[johnnyonline]

Adds a configurable minimumPrice parameter to the auction contract that automatically deactivates auctions when the price decays below the specified threshold.

• Added minimumPrice state variable (scaled to 1e18)
• Added setMinimumPrice() governance function to configure the minimum price
• Modified _price() to return 0 when current price falls below minimum, making the auction inactive
• Updated isActive() to rely on price > 0, which now accounts for minimum price
• Modified _kick() to allow re-kicking when price is below minimum (even if auction length hasn't fully elapsed)
• Removed redundant auction expiry/kicked checks from _take() - these are now handled implicitly by _price() returning 0 (causing _getAmountNeeded() to return 0 and revert with "zero needed") when the auction is expired, not kicked, or below minimum price