feat: multi-clinic Temetro Network with per-clinic identity (v0.8.0)

The relay is now multi-clinic. Each clinic authenticates to the /hub
namespace by signing a challenge with its own Ed25519 clinic signing key
(a per-clinic identity, not a shared RELAY_TOKEN), and the relay routes
every device response back to only the clinic that originated the request
(keyed by requestId) — so clinics never see each other's traffic.

Backend:
- clinic_signing_keys.network_enabled + GET/PUT /api/signing/network
  (owner/admin) to join/leave the network.
- relay-client keeps one authenticated hub connection per network-enabled
  org (connectOrg/disconnectOrg, hubs map keyed by orgId); emitToWallet/
  sendToWallet take orgId; offline flush is org-scoped.
- Wallet import/push return 409 until a clinic joins.
- RELAY_TOKEN is now optional/legacy (open relay needs no shared secret).

Frontend:
- "Join Temetro Network" toggle in Settings → Signing, localized in all
  five languages (en, fr, de, so, ar).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/db/schema/signing.ts

@@ -1,4 +1,4 @@
-import { pgTable, text, timestamp } from "drizzle-orm/pg-core";
+import { boolean, pgTable, text, timestamp } from "drizzle-orm/pg-core";
 
 import { organization } from "./auth.js";
 
@@ -16,6 +16,11 @@ export const clinicSigningKeys = pgTable("clinic_signing_keys", {
   fingerprint: text("fingerprint").notNull(),
   // Encrypted (lib/crypto.ts) hex of the Ed25519 private key.
   privateKeyEnc: text("private_key_enc").notNull(),
+  // Whether this clinic has joined the Temetro Network relay ("Join Temetro
+  // Network" in Settings → Signing). Off by default: only when enabled does the
+  // backend open this clinic's relay hub connection and expose wallet features.
+  // The relay identity *is* this signing key, so the flag lives on the same row.
+  networkEnabled: boolean("network_enabled").notNull().default(false),
   createdAt: timestamp("created_at").defaultNow().notNull(),
   rotatedAt: timestamp("rotated_at"),
 });