Expose collection freshness in school bot endpoint

Add collection_updated_at and collection_book_count to the
/v1/school/{id}/bot response. The chatflow uses this to warn
schools with stale (>3 years old) collection data.

Author: hardbyte

app/api/schools.py

@@ -224,14 +224,22 @@ async def get_school_bookbot_type(
     or their own, and which "experiments" the school has activated, if any.
     Used for the publicly-accessible Bookbot chat links.
     """
-    # dependency will automatically 404 if school doesn't exist
-    return {
+    result = {
         "name": school.name,
         "type": school.bookbot_type,
         "experiments": school.info["experiments"],
         "supporter": school.subscription is not None and school.subscription.is_active,
     }
 
+    if school.collection:
+        result["collection_updated_at"] = school.collection.updated_at.isoformat()
+        result["collection_book_count"] = school.collection.book_count
+    else:
+        result["collection_updated_at"] = None
+        result["collection_book_count"] = 0
+
+    return result
+
 
 @router.get("/school/{wriveted_identifier}/bookbot", response_model=SchoolBookbotInfo)
 async def get_school_bookbot_info(