fix(personal): scope RAG file delete to the caller's own upload dir (#4602)

The DELETE /api/personal/file disk-delete containment check used the
shared PERSONAL_UPLOADS_DIR root, so one admin could delete another
user's personal upload by passing its path (uploads are partitioned per
owner under <root>/<owner>/). Confine the check to the caller's own
per-owner subdir via _personal_upload_dir_for_owner(owner). RAG removal
and listing exclusion are unchanged (they still serve non-upload indexed
sources). Adds a regression test for the cross-owner case.

routes/personal_routes.py

@@ -358,11 +358,13 @@ def setup_personal_routes(personal_docs_manager, rag_manager, rag_available):
                 except Exception as e:
                     logger.warning(f"RAG removal failed for {filepath}: {e}")
 
-            # Delete file from disk if it's in uploads dir
+            # Delete file from disk if it's in the caller's own uploads dir.
+            # Scope to the per-owner subdir, not the shared uploads root, so one
+            # admin can't delete another user's personal files by path.
             deleted_from_disk = False
             try:
                 abs_target = os.path.realpath(filepath)
-                base_abs = os.path.realpath(UPLOADS_DIR)
+                base_abs = os.path.realpath(_personal_upload_dir_for_owner(owner, create=False))
                 in_uploads = (
                     abs_target == base_abs
                     or os.path.commonpath([abs_target, base_abs]) == base_abs