IAV-1402/fix(title-generation): return greeting concept instead of echoing the word

When users send a greeting (e.g. "salut", "hello"), the title generation endpoint was echoing the word itself as the title. Updated TITLE_SYSTEM_PROMPT to instruct the model to return the noun representing the greeting concept in the user's language (e.g. "Salut" → "Salutation", "Hello" → "Greeting"), consistently across all languages.