Add network monitoring, dashboard, and Home Assistant MQTT publishing

Keep /alive unchanged and serve a dashboard at / with status cards,
latency graphs, an event timeline, and a Wi-Fi section.

Background monitors on configurable intervals: TCP reachability,
DNS resolution, public IP change detection, host reboot detection,
Wi-Fi link quality, and Wi-Fi scan of networks in range via iw.
The Wi-Fi interface is selected with WIFI_INTERFACE; the page lists
visible interfaces and reports when none is available. Scan results
stay in memory, other metrics go to SQLite on a mounted volume.

Optional MQTT publishing with Home Assistant discovery groups all
sensors under one device per host.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TdvJYoY8kc8bmBck89U2i6

config.py

@@ -0,0 +1,88 @@
+import os
+import re
+import socket
+
+
+def _int(name, default):
+    raw = os.environ.get(name)
+    if raw is None or raw.strip() == "":
+        return default
+    try:
+        return int(raw)
+    except ValueError:
+        return default
+
+
+def _float(name, default):
+    raw = os.environ.get(name)
+    if raw is None or raw.strip() == "":
+        return default
+    try:
+        return float(raw)
+    except ValueError:
+        return default
+
+
+def _list(name, default):
+    raw = os.environ.get(name, default)
+    return [item.strip() for item in raw.split(",") if item.strip()]
+
+
+def _slug(value):
+    return re.sub(r"[^a-z0-9_-]+", "_", value.lower()).strip("_") or "device"
+
+
+class Config:
+    def __init__(self):
+        env = os.environ
+        hostname = socket.gethostname()
+
+        self.port = _int("PORT", 9999)
+        self.log_level = env.get("LOG_LEVEL", "INFO")
+        self.db_path = env.get("DB_PATH", "/data/healthcheck.db")
+        self.sample_retention_days = _int("SAMPLE_RETENTION_DAYS", 7)
+
+        self.device_name = env.get("DEVICE_NAME", hostname)
+        self.device_id = _slug(env.get("DEVICE_ID", self.device_name))
+
+        self.reach_targets = _list("REACH_TARGETS", "1.1.1.1:443,8.8.8.8:443,9.9.9.9:443")
+        self.reach_interval = _int("REACH_INTERVAL", 30)
+        self.reach_timeout = _float("REACH_TIMEOUT", 3.0)
+
+        self.dns_name = env.get("DNS_NAME", "cloudflare.com")
+        self.dns_interval = _int("DNS_INTERVAL", 60)
+
+        self.public_ip_urls = _list(
+            "PUBLIC_IP_URLS",
+            "https://api.ipify.org,https://ifconfig.me/ip,https://icanhazip.com",
+        )
+        self.public_ip_interval = _int("PUBLIC_IP_INTERVAL", 300)
+        self.public_ip_timeout = _float("PUBLIC_IP_TIMEOUT", 5.0)
+
+        self.uptime_interval = _int("UPTIME_INTERVAL", 60)
+
+        self.wifi_interface = env.get("WIFI_INTERFACE", "").strip()
+        self.wifi_link_interval = _int("WIFI_LINK_INTERVAL", 30)
+        self.wifi_scan_interval = _int("WIFI_SCAN_INTERVAL", 60)
+
+        self.mqtt_host = env.get("MQTT_HOST", "").strip()
+        self.mqtt_port = _int("MQTT_PORT", 1883)
+        self.mqtt_username = env.get("MQTT_USERNAME", "")
+        self.mqtt_password = env.get("MQTT_PASSWORD", "")
+        self.mqtt_discovery_prefix = env.get("MQTT_DISCOVERY_PREFIX", "homeassistant")
+
+    def public(self):
+        return {
+            "device_name": self.device_name,
+            "device_id": self.device_id,
+            "reach_targets": self.reach_targets,
+            "reach_interval": self.reach_interval,
+            "dns_name": self.dns_name,
+            "dns_interval": self.dns_interval,
+            "public_ip_interval": self.public_ip_interval,
+            "wifi_interface": self.wifi_interface,
+            "wifi_link_interval": self.wifi_link_interval,
+            "wifi_scan_interval": self.wifi_scan_interval,
+            "sample_retention_days": self.sample_retention_days,
+            "mqtt_enabled": bool(self.mqtt_host),
+        }