[BUG]: Fix missing fmt.Sprintf argument in debate bot opening-statement prompt

### Bug Description

The opening-statement prompt in backend/services/debatevsbot.go contains 13 %s format placeholders but supplies only 12 arguments to fmt.Sprintf.

This causes Go’s vet check to fail and may insert the formatting artifact %!s(MISSING) into prompts sent to the Gemini model. As a result, the AI receives malformed instructions, potentially reducing the quality of its opening statements.

### Steps to Reproduce

1. Clone and open the DebateAI repository.

2. Navigate to the backend directory:
cd backend

3. Run the services test command:
go test ./services

4. Observe the following error:
services/debatevsbot.go:191:4: fmt.Sprintf format %s reads arg #13, but call has 12 args
FAIL arguehub/services

5. Inspect the opening-statement fmt.Sprintf call in backend/services/debatevsbot.go and confirm that the format string has more placeholders than supplied arguments.

### Logs and Screenshots

### Environment Details

_No response_

### Impact

Medium - Feature works but has issues

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Open backend/services/debatevsbot.go and inspect the opening-statement fmt.Sprintf call around the reported line, comparing its 13 placeholders with the supplied arguments. Run cd backend && go test ./services; done means the format-check failure is resolved and the services tests pass without a missing-format artifact.

索引モデルが issue の本文から書いたものです。