thermal+ha: don't lose boot-time pages; stop HA gracefully

van-thermal fires its first sample seconds after start, and the Pi often
boots hot — order after network-online.target and retry network failures
in the Pushover send path (3 attempts, 15s apart) so that page survives
DNS not being up yet. Non-200 responses still don't retry.

The Quadlet gets StopTimeout=120: podman's default 10s window SIGKILLed
HA mid-flush and the recorder complained about an unclean sqlite shutdown
on every start. Verified clean after a full stop/start cycle.

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

power/van-thermal

@@ -73,7 +73,7 @@ def load_creds(cfg):
         return None
 
 
-def pushover(cfg, title, message, priority=0):
+def pushover(cfg, title, message, priority=0, attempts=3, retry_delay=15):
     creds = load_creds(cfg)
     if not creds:
         log(f"pushover skipped (no credentials): {title} — {message}", "warn")
@@ -84,15 +84,22 @@ def pushover(cfg, title, message, priority=0):
         "message": message, "priority": priority,
     }).encode()
     req = urllib.request.Request("https://api.pushover.net/1/messages.json", data=data)
-    try:
-        with urllib.request.urlopen(req, timeout=10) as resp:
-            ok = resp.status == 200
-        if not ok:
-            log(f"pushover HTTP {resp.status}", "warn")
-        return ok
-    except Exception as e:
-        log(f"pushover send failed: {e}", "warn")
-        return False
+    # Retry network failures (DNS not up yet at boot, WAN flap) — they block the
+    # sample loop briefly, which is fine at this cadence. A non-200 means Pushover
+    # rejected the request (bad token etc.); retrying won't change that.
+    for attempt in range(1, attempts + 1):
+        try:
+            with urllib.request.urlopen(req, timeout=10) as resp:
+                ok = resp.status == 200
+            if not ok:
+                log(f"pushover HTTP {resp.status}", "warn")
+            return ok
+        except Exception as e:
+            last = attempt == attempts
+            log(f"pushover send failed ({attempt}/{attempts}): {e}", "warn")
+            if not last:
+                time.sleep(retry_delay)
+    return False
 
 
 def load_config():