Birthday sync-device list + guard group-visible against birthday calendar

- new BirthdaySyncDevice table + /api/birthdays/sync-report & /devices so the
  web can show "birthdays come from these devices" (iOS reports its device on
  each Contacts sync)
- settings: reject setting a birthday calendar as the group-visible calendar
  (it can still be shared directly)
- web: Settings > Calendars > Birthdays shows the device list; exclude birthday
  calendars from the group-visible picker

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/birthdays_router.py

@@ -0,0 +1,77 @@
+"""Birthday sync device tracking.
+
+The iOS app reports, after each Contacts birthday sync, which device it was and
+how many birthdays it manages. The web shows this as a "birthdays come from
+these devices" list. Birthday events themselves are ordinary local events
+(see local_router); this router only tracks the sync sources.
+"""
+
+from datetime import datetime, timezone
+
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel
+from sqlalchemy.orm import Session
+
+import models
+from auth import get_current_user
+from database import get_db
+
+router = APIRouter()
+
+
+class SyncReport(BaseModel):
+    device_id: str
+    device_name: str
+    count: int = 0
+
+
+@router.post("/sync-report")
+def report_sync(
+    data: SyncReport,
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    """Upsert the (user, device) sync record after a Contacts birthday sync."""
+    row = (
+        db.query(models.BirthdaySyncDevice)
+        .filter(
+            models.BirthdaySyncDevice.user_id == current_user.id,
+            models.BirthdaySyncDevice.device_id == data.device_id,
+        )
+        .first()
+    )
+    now = datetime.now(timezone.utc).isoformat()
+    name = (data.device_name or "Gerät")[:120]
+    if row is None:
+        db.add(models.BirthdaySyncDevice(
+            user_id=current_user.id, device_id=data.device_id,
+            device_name=name, last_sync=now, count=data.count,
+        ))
+    else:
+        row.device_name = name
+        row.last_sync = now
+        row.count = data.count
+    db.commit()
+    return {"ok": True}
+
+
+@router.get("/devices")
+def list_devices(
+    db: Session = Depends(get_db),
+    current_user: models.User = Depends(get_current_user),
+):
+    rows = (
+        db.query(models.BirthdaySyncDevice)
+        .filter(models.BirthdaySyncDevice.user_id == current_user.id)
+        .order_by(models.BirthdaySyncDevice.last_sync.desc())
+        .all()
+    )
+    return [
+        {
+            "device_id": r.device_id,
+            "device_name": r.device_name,
+            "last_sync": r.last_sync,
+            "count": r.count,
+        }
+        for r in rows
+    ]