Establish Railiance Fabric graph model

fabric/dependencies/the-custodian-state-hub-postgresql.yaml

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+apiVersion: railiance.fabric/v1alpha1
+kind: DependencyDeclaration
+metadata:
+  id: the-custodian.state-hub.needs-postgresql
+  name: State Hub PostgreSQL dependency
+  owner: the-custodian
+  repo: the-custodian
+  domain: custodian
+  source_links:
+    - label: Seed dependency declaration
+      path: fabric/README.md
+spec:
+  lifecycle: active
+  environments: [all]
+  consumer_service_id: the-custodian.state-hub
+  requires:
+    capability_type: postgresql-database-service
+    capability_id: railiance-platform.cnpg.postgresql
+  interface:
+    type: database-connection
+    version_constraint: ">=16 <17"
+  auth:
+    method: database_role
+  criticality: critical
+  data_classification: confidential
+  fallback:
+    mode: none
+    description: State Hub cannot persist coordination state without PostgreSQL.