feat(01-02): costanti dominio, schemas Pydantic, FormatSelector

- backend/constants.py: CANVA_FIELDS (33 col locked), PERSUASION_DISTRIBUTION (13), SCHWARTZ_DISTRIBUTION (13)
- backend/schemas/calendar.py: CalendarSlot, CalendarRequest, CalendarResponse
- backend/schemas/generate.py: SlideContent, GeneratedPost, TopicResult, GenerateRequest, PostResult, GenerateResponse
- backend/data/format_mapping.json: matrice 6 tipi x 5 livelli (30 combinazioni)
- backend/services/format_selector.py: FormatSelector con select_format e fallback PAS
- fix .gitignore: backend/data/prompts/ e format_mapping.json non erano ignorabili

.gitignore

@@ -31,8 +31,14 @@ Thumbs.db
 *.log
 npm-debug.log*
 
-# Data (local dev)
-data/
+# Data (local dev - runtime generata, non sorgente)
+# Nota: backend/data/prompts/ e backend/data/format_mapping.json sono file
+# sorgente e DEVONO essere committati.
+# Solo la directory data/ a radice del progetto (dati Supabase, ecc.)
+/data/
 *.db
 *.sqlite
+# Sottocartelle runtime in backend/data (output generati, campagne, config utente)
 backend/data/outputs/
+backend/data/campaigns/
+backend/data/config/