feat: expose signed platform health status

migrations/000039_platform_health_snapshot.up.sql

@@ -0,0 +1,18 @@
+BEGIN;
+
+CREATE TABLE platform_health_snapshot (
+    id            bigserial PRIMARY KEY,
+    public_id     uuid NOT NULL UNIQUE DEFAULT gen_random_uuid(),
+    schema_name   varchar(64) NOT NULL CHECK (schema_name='platform-server-health-v1'),
+    status        varchar(16) NOT NULL CHECK (status IN ('healthy','warning','critical')),
+    observed_at   timestamptz NOT NULL,
+    received_at   timestamptz NOT NULL DEFAULT now(),
+    checks        jsonb NOT NULL,
+    body_digest   char(64) NOT NULL UNIQUE,
+    CONSTRAINT platform_health_checks_array CHECK (jsonb_typeof(checks)='array')
+);
+
+CREATE INDEX platform_health_snapshot_observed_idx
+    ON platform_health_snapshot(observed_at DESC, id DESC);
+
+COMMIT;