Generate an event poster (PNG) using the SurgePix API, returning a download URL. Use when the user says "generate poster", "生成海报", "做个海报", "make a poster", "create a poster", or wants a poster from event info (name, date, venue). Language: match user's conversation language for all on-poster text (E
Generate an event poster (PNG) using the SurgePix API, returning a download URL. Use when the user says "generate poster", "生成海报", "做个海报", "make a poster", "create a poster", or wants a poster from event info (name, date, venue). Language: match user's conversation language for all on-poster text (English prompt → English event-name/venue/description).