This will make it easier to migrate to FastAPI.

PR #11991: Refactors lists_json_post in prep for fastapi
Changes Made:

1. Extracts logic into new process_new_list static method
2. Changes self.process_seeds() → lists_json.process_seeds() (both are identical - call ListRecord.normalize_input_seed)
3. Spam check now raises ValueError("Spam list") caught by caller instead of directly calling self.forbidden()
   Risks: None identified
   The refactor is safe:

• Both process_seeds implementations are identical
• Error handling behavior is preserved
• Uses same patterns as existing code (process_seeds_update already uses lists_json.process_seeds)
• The new exception handling flow for spam produces the same result
  The PR is a straightforward extraction/refactor with no functional changes.