docs(agile): initialize docs

docs/architecture/database-architecture.md

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+# 4. Database Architecture (Revised with Prisma)
+
+The database schema will be defined declaratively using the Prisma schema language in a single source of truth file: `prisma/schema.prisma`. Prisma will be responsible for generating SQL migrations and providing a type-safe client for all database interactions.
+
+## Prisma Schema (`prisma/schema.prisma`)
+
+```prisma
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+generator client {
+  provider = "prisma-client-js"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+// User-friendly metadata associated with a public key.
+model Registry {
+  publicKey   String    @id
+  citizenName String
+  avatarUrl   String?
+  isActive    Boolean   @default(true)
+  createdAt   DateTime  @default(now())
+}
+
+// The core, immutable data model.
+// Once a row is written, it is never updated or deleted.
+model LedgerEntry {
+  id                 BigInt    @id @default(autoincrement())
+  previousHash       String?
+  entryHash          String    @unique
+  citizenPublicKey   String
+  signature          String
+  actionType         String
+  payload            Json? // The actual data for the action, e.g., cell details
+  createdAt          DateTime  @default(now())
+
+  @@index([citizenPublicKey])
+  @@index([payload], name: "idx_ledger_payload_entity") // Example for indexing specific payload keys
+}
+```
+
+## Migrations Workflow
+
+Database migrations will be managed exclusively by Prisma Migrate.
+1.  **Development:** After any change to `schema.prisma`, a new migration will be created and applied using the command:
+    ```bash
+    npx prisma migrate dev --name <descriptive_migration_name>
+    ```
+2.  **Production:** Migrations will be applied during the deployment process using:
+    ```bash
+    npx prisma migrate deploy
+    ```
+This ensures that the database schema is always in sync with the Prisma schema definition.
+
+---