Restore per-leader routing with configurable email addresses

Previously routing was hardcoded (Andrea for indoor, Barbara for outdoor).
Then it was replaced with a flat ADMIN_EMAILS list which lost the routing.
This commit restores routing via three separate env vars:

  ADMIN_EMAIL_INDOOR  — indoor leader, To when indoor days are booked
  ADMIN_EMAIL_OUTDOOR — outdoor leader, To when outdoor days are booked
  ADMIN_EMAIL_CC      — always Cc'd (comma-separated for multiple)

For testing, set all three to your own address so no real leader gets mail.

Changes:
- Config: replaced admin_emails with admin_email_indoor/outdoor/cc fields
- AdminNotifier: replaced admin_emails param with indoor_email/outdoor_email/
  cc_emails; _recipients_for() restored as an instance method using these
- main.py: wires the three new config fields into AdminNotifier
- .env.example: documents the three new variables with production defaults
- Tests: fixture updated to use new params

https://claude.ai/code/session_01HaUFs7SaLD5SoiuGCY27Tw

tests/test_notifier.py

@@ -15,7 +15,9 @@ def notifier():
         password="secret",
         use_tls=True,
         from_email="agent@example.com",
-        admin_emails=["to@example.com", "cc1@example.com", "cc2@example.com"],
+        indoor_email="andrea@example.com",
+        outdoor_email="barbara@example.com",
+        cc_emails=["markus@example.com"],
     )