refactor: make NostrFilterGroup accept filters directly, add fromReq for REQ arrays

[Thunder-Blaze]

Description

The FIXME at line 266 notes that NostrFilterGroup::unwrapped() should be the default constructor. Currently, the main constructor expects a full REQ message array (["REQ", "subId", {filter}, ...]) and callers that just have filter objects must go through the unwrapped() static factory — which internally builds a fake REQ array wrapper to call the same constructor.

This refactoring:

1. Makes the primary constructor accept filter objects directly (single object or array of objects).
2. Moves the REQ-array parsing into a new fromReq() static method — used only by RelayIngester when parsing actual client REQ messages.
3. Removes the unwrapped() static method.

Issue

• FIXME in src/filters.h file on line number 266

Testing

• The application builds and runs as expected

[hoytech]

Thanks!