failover: ipv4.never-default + per-WAN probe scheduling; tune probe cadence

van-failover is now the sole owner of default-route selection: netplan/NM
profiles get ipv4.never-default so NM's own DHCP/lease renewals stop
reinstalling competing default routes (was racing van-failover's enforce_route
on Starlink's 16s lease). set_profile_metric() also pins never-default on
NM-managed connections it doesn't own the netplan source for (e.g. cellular),
and nm_gateway() falls back to the raw DHCP4 lease's `routers` option since NM
stops populating IP4.GATEWAY once never-default is set.

Probing moved from one shared round to independent per-WAN retry scheduling,
so a flaky WAN retries on its own clock instead of dragging healthy WANs into
extra probes or throttling a failing one to the slow steady-state cadence.

config.json: probe_interval 4->60, fail/ok_threshold 3/2->2/1, single probe
URL — verified live across this reboot (cellular took ~2min after NM reported
"activated" to actually pass traffic; van-failover correctly withheld/
deprioritized the default route until then, then installed it automatically).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

failover/config.json

@@ -1,12 +1,10 @@
 {
-  "probe_interval": 4,
+  "probe_interval": 60,
   "probe_timeout": 3,
-  "fail_threshold": 3,
-  "ok_threshold": 2,
+  "fail_threshold": 2,
+  "ok_threshold": 1,
   "probe_urls": [
-    "http://connectivity-check.ubuntu.com/",
-    "http://www.gstatic.com/generate_204",
-    "http://cp.cloudflare.com/"
+    "http://connectivity-check.ubuntu.com/"
   ],
   "wans": [
     { "name": "wifi",     "device": "wlan0",           "metric": 100 },