refactor(shared): strip cryptographic device provisioning (now telemetry-only)

devices rows no longer carry public_key for crypto purposes. The whole
per-device key API surface (provisionNewDevice, loadDevicePrivateKey,
saveDevicePrivateKey, forgetDevicePrivateKey, restoreDeviceFromServerRecord)
is removed; registerDevice now records {name, platform} only. SQL drops the
NOT NULL on devices.public_key so future telemetry rows can omit it.

Note: SQL not applied locally - push via pnpm prod:migrate when ready.

packages/shared/src/chat/messages.ts

@@ -77,11 +77,13 @@ export async function listConversationDeviceKeys(
     .in('user_id', memberIds);
   if (dErr) throw dErr;
 
-  return (devices ?? []).map((d) => ({
-    deviceId: d.id,
-    userId: d.user_id,
-    publicKey: pgHexToBytes(d.public_key),
-  }));
+  return (devices ?? [])
+    .filter((d): d is typeof d & { public_key: string } => d.public_key !== null)
+    .map((d) => ({
+      deviceId: d.id,
+      userId: d.user_id,
+      publicKey: pgHexToBytes(d.public_key),
+    }));
 }
 
 export interface SendMessageParams {