Depends on: #100
Migrate all bounty write operations from REST to GraphQL mutations.

Files to modify

• hooks/use-bounty-mutations.ts

Tasks

• useCreateBounty → use useCreateBountyMutation from generated hooks
• useUpdateBounty → use useUpdateBountyMutation (note: backend expects id inside input, not as a separate arg)
• useDeleteBounty → use useDeleteBountyMutation
• useClaimBounty → this mutation doesn't exist on the backend yet; either:
  • Remove this hook until the backend implements claimBounty, or
  • Use updateBounty with status: 'IN_PROGRESS' as a workaround
• Preserve optimistic update logic (the existing onMutate/onError/onSettled pattern)
• Update query key invalidation to use the new generated query key constants

Acceptance Criteria

• Create, update, and delete bounties all work via GraphQL mutations
• Optimistic updates still function correctly
• Cache invalidation still triggers list refreshes