feat: Add embedMarkdown to upload response and document attachment format

The upload response now includes an embedMarkdown field with the correct
Outline attachment syntax ([name size](/api/attachments.redirect?id=...))
so callers can insert it directly into documents for native card rendering.

IMPLEMENTATION.md updated with storage backend details, auth header,
and embedding format documentation.

server.py

@@ -684,16 +684,25 @@ async def upload_endpoint(request: Request) -> JSONResponse:
 
         logger.info("Upload to storage completed successfully")
 
-        # 4. Return success and attachment metadata
+        # 4. Return success with attachment metadata and embedding markdown
+        att_name = attachment_data.get("name", filename)
+        att_size = attachment_data.get("size", size)
+        att_url = attachment_data.get("url", "")
+
+        # Outline renders attachment cards when the link text is "filename size"
+        # and the href is the relative /api/attachments.redirect path.
+        embed_markdown = f"[{att_name} {att_size}]({att_url})"
+
         return JSONResponse(
             {
                 "ok": True,
                 "data": {
                     "id": attachment_data.get("id"),
-                    "name": attachment_data.get("name"),
-                    "size": attachment_data.get("size"),
+                    "name": att_name,
+                    "size": att_size,
                     "contentType": attachment_data.get("contentType"),
-                    "url": attachment_data.get("url"),
+                    "url": att_url,
+                    "embedMarkdown": embed_markdown,
                 },
             }
         )