Fix follow-up detection: add prepositional starters for context continuity

app/services/rag_pipeline.py

@@ -203,7 +203,12 @@ def _expand_query_locally(message: str) -> List[str]:
 # last user message from history. Used ONLY for retrieval — the LLM gets the
 # original message + history for natural conversation flow.
 _FOLLOWUP_PRONOUNS = {"them", "then", "it", "this", "that", "those", "these", "its"}
-_FOLLOWUP_STARTERS = {"and ", "also ", "what about ", "how about ", "how to "}
+_FOLLOWUP_STARTERS = {
+    "and ", "also ", "what about ", "how about ", "how to ",
+    # Prepositional modifiers — almost always continue the previous topic
+    # e.g. "during probation" after "leaves per annum" = "leaves during probation"
+    "during ", "after ", "before ", "for ", "without ", "on ", "in ", "if ",
+}
 
 
 def _resolve_followup(message: str, history: List[Dict[str, str]]) -> str: