feat: add report_date to monitoring sessions and update related functionality

fix: chart properly renders centered

backend/models.py

@@ -309,6 +309,11 @@ class MonitoringSession(Base):
     period_start_hour = Column(Integer, nullable=True)
     period_end_hour   = Column(Integer, nullable=True)
 
+    # For day sessions: the specific calendar date to use for report filtering.
+    # Overrides the automatic "last date with daytime rows" heuristic.
+    # Null = use heuristic.
+    report_date = Column(Date, nullable=True)
+
     # Snapshot of device configuration at recording time
     session_metadata = Column(Text, nullable=True)  # JSON