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/db/migrate.ts

@@ -54,6 +54,19 @@ export function runMigrations(db: Database.Database): void {
       columns: [
         { name: 'is_admin', type: 'INTEGER DEFAULT 0' }
       ]
+    },
+    {
+      name: 'users',
+      columns: [
+        { name: 'home_instance', type: 'TEXT' },
+        { name: 'replicated_instances', type: "TEXT DEFAULT '[]'" }
+      ]
+    },
+    {
+      name: 'instance_settings',
+      columns: [
+        { name: 'instance_name', type: "TEXT DEFAULT 'Backspace'" }
+      ]
     }
   ];