Make Fetcher Protocol generic on the media type

[IrishPrime]

Parameterize Fetcher on the media subtype it handles so subclass-specific implementations (currently TVMaze, future fetchers for other media types) can declare narrower fetch signatures without failing type checks.

The previous declaration:

class Fetcher(Protocol):
    def fetch(self, media: Media) -> None: ...

required every implementation to accept any Media. TVMaze.fetch narrows to TV, which means it does not satisfy the protocol; passing a generic Media to TVMaze.fetch would be a type error. mypy was (correctly) flagging this with a [arg-type] error at the PROCESSOR_FACTORIES dict literal.

The new declaration uses Python 3.12 generic class syntax:

class Fetcher[M: Media](Protocol):
    def fetch(self, media: M) -> None: ...

Processor and ProcessorFactory are parameterized to match, so the type relationship between a factory's media_type and its fetcher is now expressed in the type system instead of being implicit. The heterogeneous PROCESSOR_FACTORIES dict uses ProcessorFactory[Any] as its value type.

Internally, Processor.process now returns M instead of Media (callers see a narrower type at no cost) and the local annotation on media was tightened from Media to M so the narrowing carries through to self.fetcher.fetch(media).

Resolve #161.