fix(prompt): require the year field so it stops intermittently dropping

The previous "title without the year" wording let the model omit the year
entirely instead of moving it to the year field, so years vanished from the UI
at random. Now explicitly require a four-digit year in the year field, name-only
in title.

backend/prompt.py

@@ -23,7 +23,7 @@ FILMS: Recommend exactly 3 films that belong to this mood. For each:
 - Title and year
 - 2-3 sentence rationale — the atmospheric, felt connection. Why does this film belong to this moment?{film_extra}
 
-The first film should feel most precisely matched. Give each film's title alone, without the year in the title text — the year has its own field.
+The first film should feel most precisely matched. For every film, always fill the "year" field with the film's four-digit release year as a number; put only the film's name in "title", with no year inside it.
 
 Output the JSON object below and nothing else: no markdown, no code fences, no text before or after it. Use straight double quotes, and escape any quote that appears inside a string value.
 {{"interpretation":"...","films":[{{"title":"...","year":1999,"rationale":"..."}}]}}"""