v1.9.1 — error propagation, overloaded_error handling

• Backend detects Anthropic overloaded_error (HTTP 529) vs permanent failure and surfaces them as { error: 'overloaded' | 'failed' } in the poll response
• On overloaded: row is deleted so analysis retries automatically on the next poll cycle
• Frontend shows 'Claude is currently overloaded. Retrying automatically...' amber banner and suppresses the generic slow warning while it's active
• Permanent failures still show the error screen with a Try again button