Add production HTTPS and proxy settings

- Session and CSRF cookies are secure-only unless DEBUG is on.
- TRUST_X_FORWARDED_PROTO honours X-Forwarded-Proto from a TLS
  terminating ingress. Without it, Django 4+ fails the CSRF origin
  check on every form POST.
- CSRF_TRUSTED_ORIGINS, SECURE_SSL_REDIRECT and SECURE_HSTS_SECONDS are
  configurable and off by default. HSTS for subdomains and preload are
  left to ungleich, since they affect all of *.ungleich.ch.

dal/settings.py

@@ -49,6 +49,20 @@ MAILERS = {
 
 ALLOWED_HOSTS = config('ALLOWED_HOSTS', cast=Csv())
 
+# --- HTTPS / proxy -----------------------------------------------------------
+# Behind a TLS-terminating proxy (e.g. a Kubernetes ingress) Django sees plain
+# HTTP. Only enable this if the proxy always sets X-Forwarded-Proto.
+if config('TRUST_X_FORWARDED_PROTO', default=False, cast=bool):
+    SECURE_PROXY_SSL_HEADER = ('HTTP_X_FORWARDED_PROTO', 'https')
+
+# Full origins (scheme included), e.g. https://account.ungleich.ch
+CSRF_TRUSTED_ORIGINS = config('CSRF_TRUSTED_ORIGINS', default='', cast=Csv())
+
+SESSION_COOKIE_SECURE = config('SESSION_COOKIE_SECURE', default=not DEBUG, cast=bool)
+CSRF_COOKIE_SECURE = config('CSRF_COOKIE_SECURE', default=not DEBUG, cast=bool)
+SECURE_SSL_REDIRECT = config('SECURE_SSL_REDIRECT', default=False, cast=bool)
+SECURE_HSTS_SECONDS = config('SECURE_HSTS_SECONDS', default=0, cast=int)
+
 INSTALLED_APPS = [
     'django.contrib.admin',
     'django.contrib.auth',

env.sample

@@ -14,6 +14,14 @@ LOG_LEVEL=INFO                      # root log level; output goes to stdout
 #DJANGO_LOG_LEVEL=INFO
 #LDAP3_LOG=False                    # ldap3 protocol logging (very verbose)
 
+# --- HTTPS / proxy ------------------------------------------------------------
+#TRUST_X_FORWARDED_PROTO=False      # True behind a TLS-terminating ingress
+#CSRF_TRUSTED_ORIGINS=""            # comma separated, e.g. https://account.ungleich.ch
+#SESSION_COOKIE_SECURE=             # default: True unless DEBUG
+#CSRF_COOKIE_SECURE=                # default: True unless DEBUG
+#SECURE_SSL_REDIRECT=False          # usually done by the ingress
+#SECURE_HSTS_SECONDS=0              # read Django's HSTS docs before raising
+
 # --- LDAP -------------------------------------------------------------------
 # Space separated; the first reachable server is used (failover).
 LDAPSERVER="ldap://ldap1.ungleich.ch ldap://ldap2.ungleich.ch"