feat: add locked new-year rollover wizard for season transitions

Promote returning students, assign officers, and clear last season's data after an automatic SQLite backup, with Docker volume path docs fixed for /app/Data.

Co-authored-by: Cursor <cursoragent@cursor.com>

docker-compose.example.yml

@@ -26,8 +26,9 @@ services:
       # Option 1: Volume-mounted secrets file (recommended for easy editing)
       - ./auth-secrets.json:/app/secrets/auth-secrets.json:ro
 
-      # Database persistence
-      - ./data:/app/data
+      # Database persistence — must use capital D (/app/Data). Linux paths are
+      # case-sensitive; ./data:/app/data will NOT persist app.db or rollover backups.
+      - ./data:/app/Data
 
       # HTTPS certificate (if needed)
       # - ./certs:/https:ro