v1.9.0 -> v1.10.0: file upload UI + attachment management

- Paperclip icon left of text input, file preview pill (image thumb or file icon)
- Conversation list shows attachment icon right of trash, opens gallery overlay
- Gallery overlay: scrollable, close (X), delete attachment per item
- DELETE /api/upload/{id} removes from SQLite + Qdrant
- PATCH /api/upload/{id}/link ties upload to conversation
- GET /api/upload/by-conversation/{id} lists attachments
- Chat accepts upload_context_id, injects [ATTACHED DOCUMENT] into system prompt
- Conversation list includes attachment_count field
- 8 new tests: upload context injection, delete, link, by-conversation, image type, attachment count

Still missing (P3): drag-and-drop upload, global attachments page, file download, batch upload

routers/conversations.py

@@ -15,8 +15,16 @@ router = APIRouter()
 async def list_conversations():
     db = get_db()
     rows = db.execute("SELECT * FROM conversations ORDER BY updated_at DESC").fetchall()
+    result = []
+    for r in rows:
+        c = dict(r)
+        attach_count = db.execute(
+            "SELECT COUNT(*) FROM upload_context WHERE conversation_id = ?", (c["id"],)
+        ).fetchone()[0]
+        c["attachment_count"] = attach_count
+        result.append(c)
     db.close()
-    return [dict(r) for r in rows]
+    return result
 
 
 @router.post("/api/conversations")