feat(server): publish a stable installation identity for client connections

The iOS app lets an account reach one server through several addresses, such as a home network
IP and a public name. Before it sends credentials to an alternative address it needs to know the
address leads to the same installation, so expose a public, credential-free GET /server-identity
returning a persisted UUID, and add the same optional instanceId to app-info with the
server-connection-identity feature flag.

The UUID lives in a singleton server_identity row shared by every replica, created on first use
and cached per process. An unreadable identity omits instanceId instead of failing app-info,
which clients rely on for feature detection. Existing routes and bodies are unchanged and the
web client does not call the new route; the iOS app does.

The migration uses IF NOT EXISTS because an unreleased build created the same table as 0109.

scripts/e2e/suite-registry.mjs

@@ -549,6 +549,24 @@ export const E2E_SUITES = Object.freeze({
       ...SHARED_DB_AND_HELPER_PATHS,
     ],
   },
+  "server-connection-identity": {
+    id: "server-connection-identity",
+    name: "Server Connection Identity",
+    timeout: 60,
+    lane: "full",
+    description: "Public installation identity and concurrent singleton persistence",
+    vitestTarget: "test/server-connection-identity.e2e-spec.ts",
+    junitOutput: `${TEST_RESULTS_DIR}/server-connection-identity-e2e-junit.xml`,
+    prepareDedicatedDatabase: true,
+    useDedicatedDatabase: true,
+    changedPaths: [
+      "server/src/modules/server-identity/**",
+      "server/src/modules/app-info/**",
+      "server/test/server-connection-identity.e2e-spec.ts",
+      ...SHARED_TYPES_PATHS,
+      ...SHARED_DB_AND_HELPER_PATHS,
+    ],
+  },
   "app-settings-oidc-contract": {
     id: "app-settings-oidc-contract",
     name: "App Settings OIDC Contract",