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

src/config.py

@@ -32,10 +32,12 @@ class Config:
     # Registration email address shown to parents
     registration_email: str = ""
 
-    # Admin notification recipients (comma-separated).
-    # First address → To; remaining addresses → Cc.
-    # Set to a single address (e.g. your own) during testing.
-    admin_emails: list = field(default_factory=list)
+    # Admin notification routing.
+    # Each leader receives mail only when a day in their group is booked.
+    # For testing, point all three to your own address.
+    admin_email_indoor: str = ""   # Indoor leader (Andrea Sigrist) — To when indoor booked
+    admin_email_outdoor: str = ""  # Outdoor leader (Barbara Gross) — To when outdoor booked
+    admin_email_cc: str = ""       # Always Cc'd (Markus Graf / admin); comma-separated if multiple
 
     # Storage
     data_dir: Path = field(default_factory=lambda: Path("data"))
@@ -61,11 +63,9 @@ class Config:
             smtp_port=int(os.getenv("SMTP_PORT", "587")),
             smtp_use_tls=os.getenv("SMTP_USE_TLS", "true").lower() == "true",
             registration_email=os.getenv("REGISTRATION_EMAIL", ""),
-            admin_emails=[
-                e.strip()
-                for e in os.getenv("ADMIN_EMAILS", "").split(",")
-                if e.strip()
-            ],
+            admin_email_indoor=os.getenv("ADMIN_EMAIL_INDOOR", ""),
+            admin_email_outdoor=os.getenv("ADMIN_EMAIL_OUTDOOR", ""),
+            admin_email_cc=os.getenv("ADMIN_EMAIL_CC", ""),
             data_dir=Path(os.getenv("DATA_DIR", "data")),
             knowledge_base_dir=Path(
                 os.getenv(