feat: implement Phase 1 server-side federation groundwork

Add multi-instance support foundation: shared federation types
(ReplicatedInstance, InstanceInfoResponse, VerifyPasswordRequest),
database schema changes (home_instance, replicated_instances on users,
instance_name on settings), public instance info endpoint, auth
registration with homeInstance and username@domain collision fallback,
password verification endpoint, and replicatedInstances sync on user
profile. Extract duplicated sanitizeUser into shared utility across
8 server files.

packages/server/src/routes/instance.ts

@@ -0,0 +1,24 @@
+import type { FastifyInstance } from 'fastify';
+import { eq } from 'drizzle-orm';
+import { getDb, schema } from '../db/index.js';
+import { config } from '../config.js';
+import type { InstanceInfoResponse } from '@backspace/shared';
+
+const BACKSPACE_VERSION = '1.0.0';
+
+export async function instanceRoutes(app: FastifyInstance): Promise<void> {
+  app.get('/api/instance/info', async (_request, reply) => {
+    const db = getDb();
+
+    const settings = db.select().from(schema.instanceSettings).where(eq(schema.instanceSettings.id, 1)).get();
+    const instanceName = settings?.instanceName ?? 'Backspace';
+
+    const response: InstanceInfoResponse = {
+      name: instanceName,
+      version: BACKSPACE_VERSION,
+      registrationOpen: config.registrationOpen,
+    };
+
+    return reply.code(200).send(response);
+  });
+}