docs: add missing MangaBaka config section and fix README drift

[bigspawn]

Why

MangaBaka (#97) was added to the README flag table and env var list, but not to either YAML example (README config.yaml block, config.example.yaml). The strategy chains also called it "enabled by default", while its default is false. The existing docs test only checks the flag table and env var list, so neither mistake failed CI.

What

• Add the mangabaka_api section to both YAML examples and fix the default wording in the strategy chains and notes.
• Fix other README statements that disagree with the code:
  • legacy CLIENT_SECRET_* env vars only work together with a config file
  • OAUTH_PORT falls back to PORT
  • favorites log lines include the media type
  • only four env vars are validated (Troubleshooting listed six)
  • the Features list now names Hato, MangaBaka and Jikan and the cron watch mode
• docs_test.go: check that every source in mappingSources() has a section in both YAML examples with the right enabled value, and that README prose uses the right "enabled/disabled by default". Both checks failed before the docs fix.
• CLAUDE.md: add the YAML examples and chain wording to the checklist for adding a source.