docs: clarify qB save path resolution

docs/service-apis.md

@@ -42,8 +42,8 @@ and exported-metainfo data, keeps the qB-local hash separately, and calculates:
 - normalized selected and selected-complete file-index sets;
 - torrent runtime state;
 - canonical path flag and content revision;
-- a save-path fingerprint based on validated path mapping, never a leaked
+- a validated qBittorrent `save_path`, retained only in the client's local
-  absolute host path.
+  normalized observation.
 
 qBittorrent 5 may expose a pure-v2 or hybrid torrent under the first 20 bytes
 of its v2 hash while separately advertising full `infohash_v1` and
@@ -56,6 +56,23 @@ All entries retain qBittorrent's stable torrent file index. Renamed/noncanonical
 content paths are rejected in v1 because they cannot be transported and merged
 without ambiguity.
 
+### Per-torrent local content roots
+
+`save_path` is not inventory, placement, or protocol data. It is qBittorrent
+metadata used only by the daemon that queried qBittorrent. Before staging a
+source, merging into an existing target, applying post-recheck permissions, or
+evicting files, that daemon maps the torrent's API-visible `save_path` through
+`qbittorrent.api_root`/`local_root`. The most-specific
+`qbittorrent.local_path_overrides` mapping wins when a nested path is exposed
+through a distinct client container mount.
+
+An ordinary nested qBittorrent path such as `/media/Data/Downloading` needs no
+per-resource configuration: it resolves beneath the configured root. A path
+outside that root, an unmapped distinct mount, or a mapped local path that is
+not a visible real directory fails the affected job before filesystem mutation.
+This check is intentionally per resource; an unrelated malformed qBittorrent
+entry cannot prevent normal resources from being staged or transferred.
+
 ### Verification guard
 
 The client captures transfer counters and state before recheck, issues recheck,

src/archive_clients/resources.py

@@ -19,6 +19,14 @@ class ResourceError(ValueError):
 
 @dataclass(frozen=True)
 class NormalizedResource:
+    """A normalized qB observation for protocol data and local file work.
+
+    ``save_path`` is qBittorrent's API-visible per-torrent content root.  It
+    is deliberately local-only: clients resolve it through their qB path
+    mapping immediately before filesystem work, and it is never serialized in
+    inventory, placements, or control protocol messages.
+    """
+
     summary: resource_pb2.ResourceSummary
     files: tuple[resource_pb2.TorrentFile, ...]
     metainfo: Metainfo