refactor(backend): use explicit types instead of var by default

STYLE.md

@@ -3,9 +3,8 @@
 ## C# code style
 
 - Code should be formatted with `dotnet format` or Rider's built-in formatter.
-- Variables should *always* be declared using `var`,
-  unless the correct type can't be inferred from the declaration (i.e. if the variable needs to be an `IEnumerable<T>`
-  instead of a `List<T>`, or if a variable is initialized as `null`).
+- Variables should always be declared with their type name, unless the type is obvious from the declaration.
+  (For example, `var stream = new Stream()` or `var db = services.GetRequiredService<DatabaseContext>()`)
 
 ### Naming