CLI: Add --database-provider flag for SQLite/SQL Server selection (G5)

[csharpfritz]

Problem

The CLI migration toolkit always emits SQL Server connection strings from the original Web.config. For portable benchmark runs and many real-world migrations, SQLite is needed. Currently, every migration run requires manual post-migration fixup:

1. Swap Microsoft.EntityFrameworkCore.SqlServer to Sqlite in csproj
2. Update connection strings in appsettings.json
3. Change UseSqlServer() to UseSqlite() in Program.cs and DbContext classes
4. Add EnsureCreated() + seed data for portable runs

This was the single largest runtime repair item in Run 43, consuming ~8 minutes of a 35-minute benchmark.

Proposed Solution

Add a --database-provider flag to the CLI (default: sqlite).

What the flag should control:

Aspect	sqlite (default)	sqlserver
NuGet package	Microsoft.EntityFrameworkCore.Sqlite	Microsoft.EntityFrameworkCore.SqlServer
Connection string	Data Source=app.db	Preserved from Web.config
DbContext config	UseSqlite(...)	UseSqlServer(...)
Program.cs	EnsureCreated() emitted	No EnsureCreated()

Implementation touches:

• ProgramCsEmitter — emit correct provider package and connection config
• ProjectScaffolder — emit correct NuGet package reference
• MigrationOptions — add DatabaseProvider property
• Program.cs CLI arg parser — add --database-provider flag

Context

• Identified as G5 gap during WingtipToys Run 43 benchmark
• See dev-docs/migration-tests/wingtiptoys/run43/report.md for details
• Three places need updating per migration: csproj, Program.cs, and DbContext OnConfiguring fallbacks

[csharpfritz]

Opened in wrong repo. Moving to upstream FritzAndFriends/BlazorWebFormsComponents.