Update preview samples to IdentityServer 8.1.0-preview.4 and show multiple storage instances

[Erwinvandervalk]

Updates the preview samples to the newly released packages and adds a demo of multiple storage instances.

Package updates

Sample	Changes
IdentityServer/v8/Storage	Duende.IdentityServer 8.1.0-preview.4, Duende.Storage.Sqlite 2.0.0-preview.3
IdentityServer/v8/UserManagement/AccountLockout	Duende.IdentityServer 8.1.0-preview.4, Duende.Storage.Sqlite 2.0.0-preview.3, Duende.UserManagement.IdentityServer8 2.0.0-preview.3
IdentityServer/v8/Spaces	The same as AccountLockout, plus Duende.Storage 2.0.0-preview.3 and Duende.Spaces 1.0.0-preview.3

API migrations

• AddSqliteInMemoryStore(...) is replaced by AddSqliteInMemory(...) / AddSqlite(...).
• AddStorage(...) now only registers a database. The Storage and Spaces samples call AddConfigurationStorage() and AddOperationalStorage() to keep IdentityServer data in it.
• User Management no longer has its own AddSqliteStore(...). AccountLockout registers the database with AddStorage(...) on the IdentityServer builder; the database file stays in the same place.
• AccountLockout calls AddDynamicSchemas() because its schema bootstrapper uses ISchemaAdmin.
• IDatabaseSchema is replaced by IStorageInstanceSchema.
• SchemaId.IdentityProvider("oidc") is replaced by SchemaId.OidcIdentityProvider.
• User Management now requires passwords of at least 15 characters, so the Spaces demo password is now Pa$$Word123456789 (seed data, development settings, README and Playwright tests).
• Spaces now uses the built-in schemas instead of copies: DefaultOidcProviderSchema for the OIDC identity provider, and BuiltInSchemas.UserProfile.Extend(DemoUserAttributes.UserName) for user profiles (email, name, given_name and family_name are built in). The built-in email is required and unique, so seeded users get <username>@example.com, and external users without an email claim get a per-user placeholder instead of the shared test@test.nl.

Multiple storage instances

The Storage sample now keeps IdentityServer configuration data and operational data in two separate named storage instances, and migrates each one separately:

.AddStorage(SampleStorage.Configuration, s => s.AddSqliteInMemory("IdentityServerStorageSample-Configuration"))
.AddStorage(SampleStorage.Operational,   s => s.AddSqliteInMemory("IdentityServerStorageSample-Operational"))
.AddConfigurationStorage(SampleStorage.Configuration)
.AddOperationalStorage(SampleStorage.Operational)

The home page shows which instance holds which data, and the README has a new "Multiple storage instances" section.

Verification

• Storage: builds with no warnings. The client credentials token request succeeds with region eu-west and fails with us-east. Signing in as alice with authorization code + PKCE creates a server-side session, which /sample/sessions lists; /sample/clients returns the stored clients.
• AccountLockout: the app starts, migrates, creates its schema and seed users, and the home page returns 200.
• Spaces: the app starts and seeds its data. The Playwright suite passes 5/5 (run with dotnet run, because dotnet test doesn't support Microsoft.Testing.Platform on the .NET 10 SDK).