Fix merkle proof generation to exclude zero-amount shares

The API endpoint was including all distribution shares when generating
merkle proofs, while the merkle tree generator script filters out
zero-amount shares. This mismatch caused "invalid merkle proof" errors
when users tried to claim tokens.

Add .gt('amount', 0) filter to the API endpoint to match the script
and ensure both generate identical merkle trees.

Author: youngkidwarrior

packages/api/src/routers/distribution.ts

@@ -23,6 +23,7 @@ export const distributionRouter = createTRPCRouter({
           .from('distribution_shares')
           .select('index, address, amount::text, user_id', { count: 'exact' })
           .eq('distribution_id', distributionId)
+          .gt('amount', 0)
           .order('index', { ascending: true })
       )