`create_seller_profile` validation gaps

[dmeiser]

Synthesized from full-repo review (KW-REVIEW-SYNTHESIS).

• Severity: Medium
• Location: src/handlers/scout_operations.py:42-103
• Summary: sellerName has no length limit, unitType is not validated against the enum, and unexpected exceptions are converted to RuntimeError instead of AppError.
• Recommendation: Add length/enum validation and normalize errors to AppError.
• Source reviews: deepseek-v4-pro