feat(13-05): implement keep_both_name() for D-03/D-05 collision naming

- keep_both_name(filename, counter) inserts counter before first extension
- Handles compound extensions (archive.tar.gz → archive (1).tar.gz)
- Handles extensionless files (README → README (1))
- Lives in services/cloud_operations as shared upload-queue helper
- All 23 upload contract + keep-both naming tests pass

backend/services/cloud_operations.py

@@ -2,7 +2,10 @@
 Cloud operations orchestration service — Phase 13.
 
 This module is the single service-layer seam for all Phase 13 cloud mutation and
-connection-lifecycle operations. It:
+connection-lifecycle operations. It also exposes shared naming utilities used by
+the upload route for keep-both collision resolution (D-03, D-05).
+
+It:
 
   - Resolves owned connections via services.cloud_items.resolve_owned_connection
   - Decrypts credentials only at the provider boundary (T-13-11)
@@ -24,6 +27,7 @@ Domain exceptions raised here:
 """
 from __future__ import annotations
 
+import pathlib
 import uuid
 from typing import Optional
 
@@ -34,6 +38,49 @@ from db.models import CloudConnection, CloudItem, CloudFolderState
 from services.cloud_items import resolve_owned_connection, ConnectionNotFound  # noqa: F401
 
 
+# ── Keep-both naming helper ───────────────────────────────────────────────────
+
+
+def keep_both_name(filename: str, counter: int) -> str:
+    """Insert a collision counter before the file extension.
+
+    D-03 / D-05: When the user chooses "Keep both" for a same-name upload
+    conflict, DocuVault renames the incoming file by inserting ` (N)` before
+    the first file extension (or at the end if no extension is present).
+
+    Examples::
+
+        keep_both_name("Report.pdf", 1)       → "Report (1).pdf"
+        keep_both_name("Report.pdf", 2)       → "Report (2).pdf"
+        keep_both_name("README", 1)           → "README (1)"
+        keep_both_name("archive.tar.gz", 1)   → "archive (1).tar.gz"
+        keep_both_name("My Document.docx", 1) → "My Document (1).docx"
+
+    The counter is always placed before the first dot in the filename so that
+    compound extensions (e.g. ``.tar.gz``) are preserved intact.
+
+    Args:
+        filename: The original filename (basename only — no path components).
+        counter:  Positive integer counter to insert (1-based by convention).
+
+    Returns:
+        The renamed filename with the counter inserted before the extension.
+    """
+    p = pathlib.PurePosixPath(filename)
+    # Use the stem from PurePosixPath but handle the suffix correctly:
+    # PurePosixPath("archive.tar.gz").suffix == ".gz" — we want all suffixes.
+    stem = p.name
+    suffix = ""
+
+    # Find the first dot that separates name from extension
+    dot_pos = stem.find(".")
+    if dot_pos > 0:
+        suffix = stem[dot_pos:]   # everything from the first dot onward
+        stem = stem[:dot_pos]     # everything before the first dot
+
+    return f"{stem} ({counter}){suffix}"
+
+
 # ── Connection status constants ───────────────────────────────────────────────
 
 STATUS_ACTIVE = "ACTIVE"