nuc/transmission-bt: document the watch folder for auto-adding torrents

scp a .torrent into /srv/media/.watchdir and Transmission auto-downloads
it. Documents the settings (watch-dir-force-generic + trash-original),
the correct active config path (--config-dir .../info), and the perms.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nuc/transmission-bt.md

@@ -15,6 +15,8 @@ the container simply has no path to the internet.
 - Downloads: `/media/downloads` (= `usb4t/media`, same dataset Jellyfin
   reads); in-progress files in `/media/.incomplete` so Jellyfin never
   scans partials
+- Watch folder: `scp` a `.torrent` into `/srv/media/.watchdir` on the host
+  and it auto-downloads (see "Watch folder" below)
 - `transmission-daemon` is `BindsTo=wg-quick@wg0.service` and binds
   peer traffic to `10.8.0.21` — three independent layers against leaks
   (no default route, unit binding, socket binding)
@@ -125,6 +127,51 @@ incus exec wireguard -- wg set wg0 peer <PUBKEY> allowed-ips 10.8.0.21/32
 incus exec wireguard -- wg-quick save wg0
 ```
 
+## Watch folder (auto-add torrents)
+
+Drop a `.torrent` into `/srv/media/.watchdir` on the host and Transmission
+auto-adds it and starts downloading — no web UI needed. The folder lives on
+the shared `usb4t/media` dataset (`/media/.watchdir` inside the container).
+
+Edit the **active** config only while the daemon is stopped (it rewrites
+`settings.json` on exit). The active file is
+`/var/lib/transmission-daemon/info/settings.json` — the daemon runs with
+`--config-dir /var/lib/transmission-daemon/info` from
+`/etc/default/transmission-daemon`, *not* the `.config` dir.
+
+```bash
+incus exec transmission-bt -- install -d -o debian-transmission \
+    -g debian-transmission -m 0775 /media/.watchdir
+incus exec transmission-bt -- systemctl stop transmission-daemon
+incus exec transmission-bt -- python3 - <<'PY'
+import json
+p = "/var/lib/transmission-daemon/info/settings.json"
+c = json.load(open(p))
+c.update({
+    "watch-dir": "/media/.watchdir",
+    "watch-dir-enabled": True,
+    "watch-dir-force-generic": True,      # poll (reliable across the shift mount)
+    "trash-original-torrent-files": True, # delete the .torrent once added
+})
+json.dump(c, open(p, "w"), indent=4)
+PY
+incus exec transmission-bt -- systemctl start transmission-daemon
+```
+
+Usage — the `.torrent` is consumed within a few seconds:
+
+```sh
+scp some.torrent root@192.168.0.3:/srv/media/.watchdir/
+```
+
+- `watch-dir-force-generic: true` makes Transmission **poll** the folder
+  instead of using inotify, so it reliably sees files written from the host
+  side across the shift-mounted share.
+- `trash-original-torrent-files: true` self-cleans the folder. A *malformed*
+  `.torrent` is not trashed and gets retried each poll — delete it by hand.
+- The watchdir is owned by `debian-transmission` (0775): root's `scp` writes
+  fine, and Transmission can read the file and remove it after adding.
+
 ## Verification
 
 ```sh