Rename to TaskTurnstile; add fluent DI builder and SqlServer table auto-initializer

[chunty]

Summary

This PR renames the project from TaskControlTower to TaskTurnstile and adds several improvements.

Changes

Rename: TaskControlTower → TaskTurnstile

• Renamed solution file (TaskControlTower.slnx → TaskTurnstile.slnx)
• Renamed project files, namespaces, and class names throughout
• Updated README.md to use turnstile metaphor consistently

New: TaskTurnstileBuilder (fluent DI builder)

• Adds a fluent builder API for configuring the backing store during service registration
• Supports AddDistributedStore(), AddDistributedStore(factory), UseTaskStateStore<T>(), and
  UseTaskStateStore(factory)

New: SqlServerTableInitializerHostedService

• Automatically creates the SQL Server distributed cache table on startup if it doesn't exist
• Avoids requiring manual schema setup or calling dotnet sql-cache create
• Decoupled TaskTurnstile.SqlServer from the core TaskTurnstile project reference

New: Directory.Build.props

• Centralises Microsoft.Extensions.* version selection per target framework (net9.0 / net10.0)