Make admin notification recipients configurable via ADMIN_EMAILS

Previously the To/Cc addresses were hardcoded in notifier.py (Andrea,
Barbara, Markus). This caused accidental emails to production contacts
during testing.

Changes:
- New ADMIN_EMAILS env var: comma-separated list of addresses.
  First address → To; remaining addresses → Cc.
- AdminNotifier now accepts admin_emails list; warns and skips if empty.
- Removed hardcoded _INDOOR_EMAIL / _OUTDOOR_EMAIL / _ADMIN_CC_EMAIL
  constants and the _recipients_for() routing method.
- Config.from_env() parses ADMIN_EMAILS into a list.
- main.py passes config.admin_emails to AdminNotifier.
- .env.example documents the new variable with production example.
- Tests: fixture updated; TestRecipientsFor removed (routing gone).

For testing: ADMIN_EMAILS=you@example.com
For production: ADMIN_EMAILS=andrea.sigrist@gmx.net,baba.laeubli@gmail.com,spielgruppen@familien-verein.ch

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

.env.example

@@ -43,6 +43,15 @@ SMTP_USE_TLS=true
 # ---------------------------------------------------------------
 REGISTRATION_EMAIL=anmeldung@example.com
 
+# ---------------------------------------------------------------
+# Admin notification recipients (comma-separated)
+# First address → To; remaining addresses → Cc.
+# For testing, set this to just your own email address.
+# Production example (indoor leader, outdoor leader, admin):
+#   ADMIN_EMAILS=andrea.sigrist@gmx.net,baba.laeubli@gmail.com,spielgruppen@familien-verein.ch
+# ---------------------------------------------------------------
+ADMIN_EMAILS=you@example.com
+
 # ---------------------------------------------------------------
 # Storage
 # ---------------------------------------------------------------