traefik: nextcloud-transport (3600s responseHeaderTimeout) — fix large-upload 504

Real root cause of Nextcloud "Unknown error during upload" on large files:
Traefik's default-transport responseHeaderTimeout=30s killed the chunk-assembly
MOVE (which takes >30s for multi-GB files) -> 504. Added a dedicated
nextcloud-transport (3600s) and pointed nextcloud-service at it, on both nodes.
Proven: 8GB upload MOVE now 201 @ 31.7s (was 504 @ 30.0s).

Rewrote the runbook: corrected cause (was mis-attributed to Cloudflare/DoH),
plus the hard-won gotcha on editing single-file-bind-mounted dynamic_conf
(in-place cp only; never mv; validate before write).

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

traefik/dynamic_conf_node01.yml

@@ -8,6 +8,11 @@ http:
         idleConnTimeout: 90s
     insecure-transport:
       insecureSkipVerify: true
+    nextcloud-transport:
+      forwardingTimeouts:
+        dialTimeout: 30s
+        responseHeaderTimeout: 3600s
+        idleConnTimeout: 90s
 
   middlewares:
 
@@ -463,7 +468,7 @@ http:
     nextcloud-service:
       loadBalancer:
         servers: [{ url: "http://192.168.99.31:11000" }]
-        serversTransport: default-transport
+        serversTransport: nextcloud-transport
 
     homeassistant-service:
       loadBalancer: