feat: add GoAlert Cloudron package (Monitoring) [#633]

Official-image wrapper of goalert/goalert 0.34.1 (digest-pinned),
single Go binary + postgresql only. Native generic OIDC wired from
the platform provider (GOALERT_OIDC_*), data-encryption key
persisted, optional first-admin basic auth. grind-stack verified:
full migration chain, serving, origin canonicalization correct.
18th package; category table reconciled to the 46-app set.

Package-Workspace/Monitoring/goalert/CloudronManifest.json

@@ -0,0 +1,22 @@
+{
+  "manifestVersion": 2,
+  "type": "app",
+  "id": "io.cloudron.goalert",
+  "title": "GoAlert",
+  "description": "Open-source on-call and alerting system by Target: escalation chains, rotation schedules, notification rules (Slack, Twilio SMS/voice, and more), heartbeat checks, and status dashboards. Single Go service with PostgreSQL storage; schema migrations run automatically on start. SSO via the Cloudron OIDC provider; optional basic-auth bootstrap for the first admin.",
+  "author": "Target Corporation / GoAlert contributors",
+  "website": "https://goalert.io/",
+  "contactEmail": "cloudron@tsys.dev",
+  "tagline": "On-call scheduling, escalations and alert routing",
+  "version": "0.34.1",
+  "healthCheckPath": "/api/v2/heartbeat",
+  "httpPort": 8081,
+  "memoryLimit": 1024,
+  "addons": {
+    "localstorage": {},
+    "postgresql": {}
+  },
+  "mediaLinks": [],
+  "changelog": "Initial Cloudron package for GoAlert 0.34.1 (official-image wrapper of goalert/goalert, digest pinned). start.sh waits for the postgresql addon, persists the data-encryption key under /app/data, maps GOALERT_DB_URL/PUBLIC_URL from the platform env, and wires native generic OIDC (GOALERT_OIDC_*) from the Cloudron OIDC provider with new-user creation enabled. Optional first-admin via GOALERT_ADMIN_USER/PASS app env vars (basic auth).",
+  "icon": "file://logo.png"
+}