Add USD price tracking for PLOT token with storyline token value calculation

[realproject7]

Summary

Add USD price display for mainnet PLOT and storyline tokens. Reference Dropcast's lib/usd-price.ts for the price fetching pattern.

Architecture

• Only need to track USD price for the mainnet PLOT token on Base mainnet
• Testnet / PL_TEST USD pricing is out of scope for this issue
• All storyline token values derive from PLOT price (storyline tokens are priced in PLOT via MCV2 bonding curve)
• USD value = storyline token price in PLOT × PLOT USD price

Implementation — reference ~/Projects/dropcast/lib/usd-price.ts

• Create lib/usd-price.ts with fallback chain:
  1. Mint Club SDK — mintclub.network('base').token().getUsdRate()
  2. GeckoTerminal API — free, covers DEX-traded tokens
  3. CoinGecko API — optional fallback (controlled by env flag)
  4. DB cache fallback — return last known price if all APIs fail
• Price cache TTL: ~2 minutes
• Display USD values on: story cards, story page, profile portfolio, token page

Acceptance Criteria

• Mainnet PLOT USD price fetched with multi-source fallback
• Storyline token USD values calculated from mainnet PLOT price
• USD values displayed alongside PLOT prices across the app where mainnet price data is available
• Price caching to avoid excessive API calls