[Feature] Add Market Cap sort option to story discovery

[realproject7]

Problem

Stories on the discover/home page can only be sorted by default order. Users who want to find trending or high-value stories have no way to sort by market cap.

Solution

Add a "Market Cap" option to the sort dropdown on the story discovery page. Stories are sorted by their token's market cap (total supply × price per token × PLOT USD price) in descending order.

Implementation

• The trade_history table already has price_per_token and total_supply for each storyline token
• PLOT USD price is available via getPlotUsdPrice()
• MCap = total_supply × price_per_token × PLOT_USD
• Add sort option to existing dropdown (alongside "Newest", "Most Plots", etc.)

Files

• Discover/home page component — add sort option
• API route for discovery — add sort=mcap query param support

Acceptance Criteria

• "Market Cap" sort option in dropdown
• Stories sorted by MCap descending
• Works on both desktop and mobile