add db ERD to docs.

docs/_sidebar.md

@@ -24,3 +24,4 @@
     * [SPIKE Nexus Automatic Recovery](architecture/charts/recovery.md)
     * [SPIKE Manual System Re-Init](architecture/charts/re-init.md)
     * [SPIKE Forced Reset](architecture/charts/reset.md)
+    * [SPIKE Nexus Backing Store](architecture/charts/database.md)

docs/architecture/charts/README.md

@@ -6,4 +6,5 @@
 * [SPIKE Nexus Root Key Provisioning](architecture/charts/day-zero.md)
 * [SPIKE Nexus Automatic Recovery](architecture/charts/recovery.md)
 * [SPIKE Manual System Re-Init](architecture/charts/re-init.md)
-* [SPIKE Forced Reset](architecture/charts/reset.md)
+* [SPIKE Forced Reset](architecture/charts/reset.md)
+* [SPIKE Nexus Backing Store](architecture/charts/database.md)

docs/architecture/charts/database.md

@@ -0,0 +1,32 @@
+## SPIKE Nexus Backing Store
+
+Here is the Entity-Relationship Diagram (ERD) for the SPIKE Nexus backing
+store:
+
+```mermaid
+erDiagram
+    admin_token {
+        integer id PK "CHECK (id = 1)"
+        blob nonce "NOT NULL"
+        blob encrypted_token "NOT NULL"
+        datetime updated_at "NOT NULL DEFAULT CURRENT_TIMESTAMP"
+    }
+
+    secrets {
+        text path PK "NOT NULL"
+        integer version PK "NOT NULL"
+        blob nonce "NOT NULL"
+        blob encrypted_data "NOT NULL"
+        datetime created_time "NOT NULL"
+        datetime deleted_time "nullable"
+    }
+
+    secret_metadata {
+        text path PK
+        integer current_version "NOT NULL"
+        datetime created_time "NOT NULL"
+        datetime updated_time "NOT NULL"
+    }
+
+    secrets ||--o| secret_metadata : "references"
+```