Add ErrorHandlingMiddleware for consistent JSON error responses

Catches all unhandled exceptions, logs them, and returns
{ "error": "An unexpected error occurred." } with HTTP 500.
Registered before all other middleware so nothing leaks through.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Budget.Api/Program.cs

@@ -126,6 +126,7 @@ using (var scope = app.Services.CreateScope())
 }
 
 app.UseForwardedHeaders();
+app.UseMiddleware<ErrorHandlingMiddleware>();
 
 app.UseDefaultFiles();
 app.UseStaticFiles();