M20: stabilize production acceptance

backend/app/services/knowledge/ragcore.py

@@ -53,15 +53,15 @@ def _deduplicate_sources(sources: list[SourceCard]) -> list[SourceCard]:
 
     RAGcore document/version UUIDs change across uploads, so they are not useful
     deduplication keys. Human-visible citation identity is the normalized title,
-    section and excerpt.
+    section. Chunks from the same unsectioned document collapse into one card; distinct
+    named sections remain independently citable.
     """
-    seen: set[tuple[str, str, str]] = set()
+    seen: set[tuple[str, str]] = set()
     unique: list[SourceCard] = []
     for source in sources:
         key = (
             source.title.strip().casefold(),
             source.section.strip().casefold(),
-            " ".join(source.excerpt.split()).casefold(),
         )
         if key in seen:
             continue