feat(content): add ModDB HTML page parser and Playwright headless service

[undead2146]

Summary

Introduces a managed headless Chromium runtime using Playwright and a robust HTML parser for scraping ModDB mod, addon, and map detail pages.

Note

This is a self-contained feature PR carved out of #265 and must be merged ahead of #265.

Motivation

ModDB pages contain rich metadata, multi-part download mirrors, screenshots, changelogs, and dependency relationships that need structured normalization for client downloads.

Changes

• Core Contracts & Models:
  • Added IWebPageParser and IPlaywrightService interfaces.
  • Added ModDBFilter, ModDBCategoryMapper, MapDetails, Comment, DownloadableFile, and FileSectionType.
• Services:
  • Implemented PlaywrightService with managed Chromium browser lifecycle, stealth configuration, and session cookie persistence.
  • Implemented ModDBPageParser for extracting structured game content, direct mirrors, and media galleries.
  • Added HtmlTextHelper for clean string normalization and markup stripping.
• DI Registration: Registered in ContentPipelineModule.
• Tests: Added 81 unit tests in ModDBPageParserTests.cs, ManagedChromiumRuntimeTests.cs, ModDBCategoryMapperTests.cs, and HtmlTextHelperTests.cs.

Verification

• All 81 unit tests and full solution tests passing across Windows, Linux, and macOS.
• Pre-tested on fork in PR feat(content): add ModDB HTML page parser and Playwright headless service undead2146/GenHub#21.