refactor: Sprint 3b — remove dead code, non-functional provider, unused fields

feat(pipeline): IIIF-native Sprint 3 — in-memory image fetch for AI analysis

fix(test): align test_prompt_loader with implementation (ValueError on unresolved vars)

fix(sprint-f3): conformité spec — IIIF langue, ai_raw.json, prompts génériques

fix(sprint-f0): fondations — lazy imports, schémas conformes, pydantic-settings

fix(vertex): corriger le routing API pour VERTEX_API_KEY — ajouter vertexai=True

feat: multi-provider dynamique — ajout Mistral/Pixtral, détection auto des providers

feat(sprint2-session-c): premier appel Google AI → master.json valide