Add strike events to activity log

Record a StrikeEvent row whenever a strike is added or removed,
and surface them in the activity log timeline with timestamp,
child name, and whether the strike was added or removed.

- New strike_events table (auto-created on startup)
- children router records prev/new strikes on every update
- GET /api/logs/strikes and DELETE /api/logs/strikes/:id endpoints
- Log view merges strike entries into the timeline (red dot,
  "✕ Strike added (2/3)" / "↩ Strike removed (1/3)")

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/activity.py

@@ -1,5 +1,5 @@
 from datetime import date, datetime, timezone
-from pydantic import BaseModel, field_serializer
+from pydantic import BaseModel, computed_field, field_serializer
 
 
 class ActivityLogCreate(BaseModel):
@@ -51,3 +51,21 @@ class TimelineEventOut(BaseModel):
         if dt.tzinfo is None:
             dt = dt.replace(tzinfo=timezone.utc)
         return dt.isoformat()
+
+
+class StrikeEventOut(BaseModel):
+    id: int
+    child_id: int
+    child_name: str
+    occurred_at: datetime
+    log_date: date
+    prev_strikes: int
+    new_strikes: int
+
+    model_config = {"from_attributes": True}
+
+    @field_serializer("occurred_at")
+    def serialize_occurred_at(self, dt: datetime) -> str:
+        if dt.tzinfo is None:
+            dt = dt.replace(tzinfo=timezone.utc)
+        return dt.isoformat()