Refactor feed import and content canonicalization workflows

[sasasin]

Summary

• Refactor feed import planning, execution, and TOML handling
• Improve content canonicalization candidate scanning, planning, execution, and cleanup
• Refine full-text probing and document fetching behavior
• Expand configuration and service test coverage

Testing

• Added and updated unit tests for configuration binding, feed import, TOML codecs, canonicalization, and HTTP fetching
• Formatting, linting, and verification not run (not requested)