feat(rag): PDF/docx ingest + chat citations

- Upload endpoint (base64 JSON, no multipart dep): extracts text from
  pdf/docx/txt/md via pypdf + python-docx, then runs the existing
  chunk/embed pipeline. Documents page uploads files straight through.
- Citations: the chat stream emits an SSE `sources` event listing the
  documents that fed the answer; the UI shows them as chips under the reply.
- Deps: pypdf, python-docx (both pure-Python, Windows-safe).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

interface/web/src/Chatbot.jsx

@@ -215,6 +215,18 @@ export function Chatbot({ conversationId, setConversationId, onConversationChang
               pendingEventType = null;
               continue;
             }
+            if (pendingEventType === "sources") {
+              try {
+                const src = JSON.parse(payload).sources;
+                setMessages(prev => {
+                  const updated = [...prev];
+                  updated[assistantIndex] = { ...updated[assistantIndex], sources: src };
+                  return updated;
+                });
+              } catch { /* ignore */ }
+              pendingEventType = null;
+              continue;
+            }
             if (pendingEventType === "done") {
               // Answer is complete; re-enable input while the backend finishes
               // slow post-processing (title, memory) on the still-open stream.
@@ -526,6 +538,16 @@ export function Chatbot({ conversationId, setConversationId, onConversationChang
                       : <span style={{ color: "#aaa" }}>Thinking...</span>
                   }
                 </div>
+                {msg.sources && msg.sources.length > 0 && (
+                  <div style={{ marginTop: "0.35rem", display: "flex", flexWrap: "wrap", gap: "0.35rem", alignItems: "center" }}>
+                    <span style={{ fontSize: "0.7rem", color: "#777" }}>Sources:</span>
+                    {msg.sources.map((s, i) => (
+                      <span key={i} style={{ fontSize: "0.72rem", color: "#8ab4ff", background: "#1a2433", border: "1px solid #2a3a52", borderRadius: "6px", padding: "0.1rem 0.45rem" }}>
+                        📄 {s}
+                      </span>
+                    ))}
+                  </div>
+                )}
                 {msg.content && editingIdx !== idx && (
                   <div style={{ display: "flex", gap: "0.25rem", marginTop: "0.25rem" }}>
                     <button