Extract Parser Strategy to enable native JSON API support

[danieldotnl]

Context

Part of the Architecture Refactoring epic (see design doc, previously PR #535).

Problem

The current parser is tightly coupled to HTML/BeautifulSoup. Adding JSON API support (a frequently requested feature) requires significant surgery.

Proposed Solution

Use the ParserFactory and pluggable parser strategies introduced in #552:

class HtmlParser:
    def parse(self, response: Response) -> HtmlContent: ...

class JsonParser:
    def parse(self, response: Response) -> JsonContent: ...

class ParserFactory:
    def get_parser(self, content_type: str) -> Parser: ...

Add JSONPath-like selector support in ValueExtractor alongside existing CSS selectors.

Dependencies

• Refactor Scraper class using Strategy pattern #552 (Refactor Scraper class using Strategy pattern) — the parser protocol must exist first

Benefits

• Native JSON API scraping without workarounds
• Clean extension point for future parsers (XML, CSV, etc.)
• CSS and JSONPath selectors work uniformly through ValueExtractor