docs(M6): aligner guide, TODO et configuration pour l'agenda théorique JSON

Met à jour la documentation et la configuration pour le jalon M6 selon
les décisions d'architecture :

- GUIDE_DEV_PYTHON.md §8 : remplace iCal/CSV par JSON avec parité de
  semaine (paire/impaire) et calendrier de vacances scolaires séparé
- TODO.md M6 : nouveaux items (WeekParityService, SchoolHolidayCalendar,
  configuration, fixtures JSON)
- .env.example : THEORETICAL_AGENDA_PATH passe en .json, ajout de
  SCHOOL_HOLIDAYS_PATH, THEORETICAL_WEEK_ANCHOR_DATE et
  THEORETICAL_WEEK_ANCHOR_TYPE
- AppSettings : 3 nouveaux champs (school_holidays_path,
  theoretical_week_anchor_date, theoretical_week_anchor_type)

Co-authored-by: opencode/coder <coder@agents.invalid>
Co-authored-by: opencode/tech-writer <tech-writer@agents.invalid>

.env.example

@@ -22,7 +22,10 @@ SYNC_PAST_DAYS=7
 SYNC_FUTURE_DAYS=30
 
 # --- Agenda théorique ---
-THEORETICAL_AGENDA_PATH=./data/theoretical.ics
+THEORETICAL_AGENDA_PATH=./data/theoretical.json
+SCHOOL_HOLIDAYS_PATH=./data/school_holidays.json
+THEORETICAL_WEEK_ANCHOR_DATE=2026-09-01
+THEORETICAL_WEEK_ANCHOR_TYPE=even
 
 # --- XMPP ---
 XMPP_ENABLED=false