Feat/lessons module ordering markdown

[rabsqueen]

feat(backend): implement lessons module with explicit ordering, markdown bodies, and reorder endpoint (#786)

• Create LessonsService and LessonsController supporting lesson creation with code-block attachments and markdown content
• Implement transactional batch reordering endpoint ensuring course outlines reflect updated indices
• Add lesson detail retrieval endpoints with associated course metadata
  closes BE-028 — Avatar & media uploads to Cloudflare R2 #781
  closes BE-029 — Account deletion & data export #782
  closes BE-030 — Session & device management #783
  closes BE-033 — Lessons module with ordering #786

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[MaryammAli]

LGTM