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.

packages/types/src/app-info.ts

@@ -17,11 +17,14 @@ export const SERVER_FEATURES = [
   "metadata-search-cache",
   "read-aloud-delivery",
   "koreader-delivery-settings",
+  "server-connection-identity",
 ] as const;
 
 export type ServerFeature = (typeof SERVER_FEATURES)[number];
 
 export interface AppInfoResponse {
+  /** Stable installation identity, absent on older servers. */
+  instanceId?: string;
   version: string;
   updateAvailable: boolean | null;
   latestVersion: string | null;
@@ -29,3 +32,10 @@ export interface AppInfoResponse {
   /** Absent on servers older than the field itself, which a client treats as an empty list. */
   features?: ServerFeature[];
 }
+
+/** Public, credential-free identity check for explicitly enrolled connection aliases. */
+export interface ServerIdentityResponse {
+  product: "bookorbit";
+  protocolVersion: 1;
+  instanceId: string;
+}