Add SPEC, PLAN, Dockerfile, compose, env example, app modules, CI workflow

.env.example

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+# FritzBox TR-064
+FRITZBOX_HOST=192.168.178.1
+FRITZBOX_PORT=49000
+FRITZBOX_USER=dyndns-updater
+FRITZBOX_PASSWORD=change-me
+
+# Cloudflare
+CLOUDFLARE_API_TOKEN=change-me
+CLOUDFLARE_ZONE_ID=change-me
+# Kommagetrennte Liste der Subdomains (ohne Proxy!), Anzahl beliebig erweiterbar
+CLOUDFLARE_RECORDS=sub1.example.org,sub2.example.org,sub3.example.org
+
+# FreeDNS (Fallback/Parallelbetrieb)
+FREEDNS_UPDATE_URL=https://freedns.afraid.org/dynamic/update.php?TOKEN
+
+# Webhook (kein Auth, rein intern)
+WEBHOOK_PORT=8090
+
+# Fallback Polling
+POLL_INTERVAL_MINUTES=15
+
+# Healthchecks (self-hosted)
+HEALTHCHECK_PING_URL=https://healthchecks.example.org/ping/change-me
+
+# Sonstiges
+TZ=Europe/Berlin
+LOG_LEVEL=INFO