fix: wire bm25_score through Citation so Natural 20 easter egg fires

Citation dataclass gains bm25_score field populated from the retrieved
chunk. chat.py serializes it. api.ts interface updated to include it.
ChatView passes :bm25-score to CitationPanel so the Nat20 threshold
check in onMounted actually has data to evaluate.

app/api/chat.py

@@ -120,6 +120,7 @@ def chat(req: ChatRequest) -> ChatResponse:
                 "doc_id": c.doc_id,
                 "page_number": c.page_number,
                 "snippet": c.snippet,
+                "bm25_score": c.bm25_score,
             }
             for c in result.citations
         ],

app/services/synthesizer.py

@@ -23,6 +23,7 @@ class Citation:
     doc_id: str
     page_number: int
     snippet: str
+    bm25_score: float
 
 
 @dataclass(frozen=True)
@@ -52,6 +53,7 @@ class Synthesizer:
                 doc_id=c.doc_id,
                 page_number=c.page_number,
                 snippet=c.text[:200],
+                bm25_score=c.bm25_score,
             )
             for c in chunks
         )

web/src/api.ts

@@ -23,6 +23,7 @@ export interface Citation {
     doc_id: string
     page_number: number
     snippet: string
+    bm25_score: number | null
 }
 
 export interface ChatResponse {

web/src/views/ChatView.vue

@@ -22,6 +22,7 @@
               :key="j"
               :citation="cite"
               :doc-title="docTitles[cite.doc_id] ?? cite.doc_id"
+              :bm25-score="cite.bm25_score ?? undefined"
             />
           </div>
         </div>