Improve `/imagine` Hugging Face errors (429, JSON errors, retries)

[rosspeili]

Problem

generate_image_from_prompt handles missing token and 503 with a friendly line; other statuses use response.text in the error string (OPSIIE_0_3_80_XP.py ~1541–1545), which can leak long JSON bodies to the terminal and TTS.

Goal

• Map common codes (429, 401, 503) to short operator messages; truncate or omit raw body in user-visible paths.
• Optional: one retry with backoff on 503 / model loading.

Acceptance criteria

• No full HF error payload printed/s spoken by default.
• docs/TROUBLESHOOTING.md updated with retry guidance if implemented.