Currently, there is a Farcaster contest bot already implemented, which programmatically sends messages to users via Neynar API: #10335

We need to setup the CW app to listen to Farcaster mentions (e.g. @contestbot) so that the bot can respond to those messages.

Relevant docs: https://docs.neynar.com/docs/listen-for-bot-mentions

Required artifacts:

• Config: Neynar webhook setup (in Neynar dev dashboard) for QA + production
• Route: Add a new webhook route in integration router which emits ContestBotMentioned event w/ Cast payload (using signature validation middleware)
• Policy: Add a new policy (or use existing FarcasterWorkerPolicy) to handle the ContestBotMentioned event (the handler is empty for now, will implement in another ticket)

Acceptance Criteria

• CW app should listen for contest bot mentions

Other Considerations

• There's separation of local, QA and production environments for the Neynar/bot configs. The bot for each environment should have a different Farcaster handle.