refactor(backend): reorganize file structure (no logic change)

[littlecoffe8]

Description

business/ and enums/ had become flat bags mixing unrelated
responsibilities, and offline data pipeline scripts were living inside the
runtime application package.

Pure file/folder reorganization of the backend Python package. No behavior
change on the served API - only file moves, renames, and import path
updates.

Code changes

• Remove app/models/ (empty, unused since project inception)
• Split the flat business/open_food_facts/ package into:
  • calculators/ (breeding_type, egg_quantity, quantity, product_type,
    pain_report, unit_pain_loader + its pain_data.csv - a data loader
    feeding the calculators, not a rendering concern)
  • panel_renderer/ (generator + html_templates - the actual rendering
    layer, nothing else)
• Rename knowledge_panel.py -> knowledge_panel_service.py
  (was easily confused with egg_knowledge_panel_generator.py)
• Split enums/open_food_facts/ into:
  • patterns/ (constant repositories - not real enums)
  • content/ (UI text catalogs)
  • enums.py keeps only actual StrEnum classes
• Move app/scripts/ -> backend/pipelines/, out of the served app
  package (offline batch scripts, unrelated to the FastAPI runtime)

How to test

pytest: 59 passed (unchanged).