Implement private Q2 signal contract and durable reference receiver

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a06ecb-456a-71c2-b41e-0755d336e883

contracts/platform-assurance.json

@@ -0,0 +1,34 @@
+{
+  "schema": "railiance-telemetry.stream.v1",
+  "stream": "railiance-platform.service-assurance",
+  "producer": "railiance-platform",
+  "recipient": "railiance-platform-operator",
+  "signals": [
+    "apps-pg.ready",
+    "apps-pg.backup",
+    "apps-pg.wal",
+    "apps-pg.restore",
+    "apps-pg.headroom",
+    "platform-pg.ready",
+    "platform-pg.backup",
+    "platform-pg.wal",
+    "platform-pg.restore",
+    "platform-pg.headroom",
+    "platform-pg-2.ready",
+    "platform-pg-2.backup",
+    "platform-pg-2.wal",
+    "platform-pg-2.restore",
+    "platform-pg-2.headroom",
+    "openbao.seal",
+    "openbao.snapshot",
+    "openbao.restore",
+    "offsite.upload",
+    "offsite.restore",
+    "eso.ready",
+    "eso.refresh",
+    "forgejo-db.restore"
+  ],
+  "max_event_age_seconds": 900,
+  "heartbeat_seconds": 900,
+  "retention_days": 30
+}