feat(backend): use [filename, page N] citation labels in RAG context (sub-phase 2.6)

Replace numeric [1] labels with [filename, page N] format in context chunks.
Update LLM prompt to instruct inline citation using bracket labels.
Enables traceable source references in generated answers.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/app/services/rag.py

@@ -95,8 +95,10 @@ class RAGService:
         for i, (chunk, meta) in enumerate(zip(chunks, metadata_list)):
             source = meta.get("filename", "unknown")
             summary = meta.get("content_summary", "")
+            page_num = meta.get("page_number")
+            citation_label = f"{source}, page {page_num}" if page_num else source
             context_parts.append(
-                f"[{i + 1}] Source: {source}\n"
+                f"[{citation_label}] Source: {source}\n"
                 f"Summary: {summary}\n"
                 f"Content: {chunk}\n"
             )
@@ -108,7 +110,8 @@ class RAGService:
             f"Answer the question using ONLY these document chunks. "
             f"Do not use any external knowledge. "
             f"Format your answer as bullet points. "
-            f"Cite the source name in [ ] for each point.\n\n"
+            f"Cite your sources inline using the exact bracket labels provided, "
+            f"e.g. [filename, page N]. Place the citation at the end of each relevant point.\n\n"
             f"Document chunks:\n{context}\n\n"
             f"Answer:"
         )