docs: clarify identity file usage and Docker persistence

- Fix incorrect filename (metrics_identity.json -> shm_identity.json)
- Add identity file structure documentation
- Add Docker/Kubernetes volume mount instructions
- Explain 401 error when identity is lost

Fixes #3

Author: btouchard

README.md

@@ -350,11 +350,36 @@ graph LR
     D[Admin Dashboard] -- Read API --> B
 ```
 
-1.  **Client:** Generates Ed25519 keys on first run. Stores identity in `metrics_identity.json`.
+1.  **Client:** Generates Ed25519 keys on first run. Stores identity in `shm_identity.json`.
 2.  **Protocol:** Sends a Heartbeat/Snapshot signed with the private key.
 3.  **Storage:** PostgreSQL stores the raw JSON payload in a `jsonb` column.
 4.  **UI:** The server parses the JSON keys dynamically to build the table and graphs.
 
+### Identity File
+
+The SDK automatically generates `shm_identity.json` on first run. This file contains:
+
+```json
+{
+  "instance_id": "550e8400-e29b-41d4-a716-446655440000",
+  "private_key": "hex-encoded-ed25519-private-key",
+  "public_key": "hex-encoded-ed25519-public-key"
+}
+```
+
+**Important for Docker/Kubernetes:** The identity file must be persisted across container restarts. Mount a volume for the `DataDir` (Go) or `dataDir` (Node.js) configuration:
+
+```yaml
+services:
+  my-app:
+    volumes:
+      - ./data:/app/data  # Persist identity file
+    environment:
+      - SHM_DATA_DIR=/app/data
+```
+
+If the identity file is lost, the instance will generate a new identity and the server will return **401 Unauthorized** for requests signed with the old key.
+
 ---
 
 ## 🛡️ Security & Privacy

sdk/golang/README.md

@@ -98,6 +98,8 @@ client, _ := shm.New(shm.Config{
 
 4. **Periodic Snapshots**: System metrics + custom metrics are sent at the configured interval
 
+> **Docker/Kubernetes:** The identity file must persist across restarts. Mount a volume for `DataDir`, otherwise a new identity will be generated and the server will reject requests with **401 Unauthorized**.
+
 ## System Metrics
 
 The SDK automatically collects:

sdk/nodejs/README.md

@@ -87,6 +87,8 @@ const client = new SHMClient({
 
 4. **Periodic Snapshots**: System metrics + custom metrics are sent at the configured interval
 
+> **Docker/Kubernetes:** The identity file must persist across restarts. Mount a volume for `dataDir`, otherwise a new identity will be generated and the server will reject requests with **401 Unauthorized**.
+
 ## System Metrics
 
 The SDK automatically collects: