docs(agile): initialize docs

docs/architecture/security-performance.md

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+# 9. Security & Performance
+
+*   **Security:**
+    *   **Authentication:** Managed via cryptographic signatures. The server is stateless regarding authentication.
+    *   **Authorization:** All API endpoints inside `server/middleware/auth.ts` will verify the incoming signature against the `publicKey` to ensure the action is performed by the legitimate owner of the key.
+    *   **Secrets:** The `DATABASE_URL` will be managed via the `.env` file, which is excluded from version control.
+*   **Performance:**
+    *   The primary performance consideration is the Raspberry Pi environment. The monolithic Nuxt/PostgreSQL stack is chosen for its low resource overhead compared to more complex microservice architectures.
+    *   Database queries will be optimized using indices as defined in the Prisma schema.
+    *   Frontend assets will be optimized by Nuxt's build process.
+
+---