refactor: Improve naming of various stuffs [skip ci]

[FAZuH]

1. Break down "feeds.url" column to "feeds.platform_id", "feeds.source_id", "feeds.items_id", and "feeds.source_url". This change is done because our assumption was that we can always get request url for feed items using id contained in "feeds.url". This assumption was violated by comick.dev, which has different id for getting comic (feed source) and chapter (feed items) information.

2. While doing point 1, we realized some feed naming is too ambiguous. We now define:

   • Platform: External service that hosts content (e.g., MangaDex, AniList)
   • Feed Source: Specific content source on a platform (e.g., "One Punch Man" on MangaDex)
   • Feed Item: Individual updates within a source (e.g., chapters, episodes)

   and made modifications accordingly