fix: carbon fallback, energy history parsing, generation dedup (v1.4.2)

- Carbon overview: fallback to compute from energy_data × emission_factors
  when carbon_emissions table is empty
- Energy history: parse start_time/end_time as datetime (was raw string → 500)
- Dashboard generation: dedup by station prefix to prevent inflated totals
  (93K → 14.8K kWh)
- Realtime window: already at 20min to cover 15min collector interval

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/v1/carbon.py

@@ -52,7 +52,10 @@ class ReportGenerate(BaseModel):
 
 @router.get("/overview")
 async def carbon_overview(db: AsyncSession = Depends(get_db), user: User = Depends(get_current_user)):
-    """碳排放总览"""
+    """碳排放总览 - 优先从carbon_emissions表读取，为空时从energy_data实时计算"""
+    from app.models.energy import EnergyData
+    from app.models.device import Device
+
     now = datetime.now(timezone.utc)
     today_start = now.replace(hour=0, minute=0, second=0, microsecond=0)
     month_start = now.replace(day=1, hour=0, minute=0, second=0, microsecond=0)
@@ -70,6 +73,52 @@ async def carbon_overview(db: AsyncSession = Depends(get_db), user: User = Depen
     month = await sum_carbon(month_start, now)
     year = await sum_carbon(year_start, now)
 
+    # Fallback: if carbon_emissions is empty, compute reduction from PV generation
+    has_carbon_data = (today["emission"] + today["reduction"] +
+                       month["emission"] + month["reduction"] +
+                       year["emission"] + year["reduction"]) > 0
+
+    if not has_carbon_data:
+        # Get grid emission factor (华北电网 0.582 kgCO2/kWh)
+        factor_q = await db.execute(
+            select(EmissionFactor.factor).where(
+                EmissionFactor.energy_type == "electricity"
+            ).order_by(EmissionFactor.id).limit(1)
+        )
+        grid_factor = factor_q.scalar() or 0.582  # default fallback
+
+        # Compute PV generation from energy_data using latest daily_energy per station
+        # Device names like AP1xx belong to station 1, AP2xx to station 2
+        # To avoid double-counting station-level data written to multiple devices,
+        # we group by station prefix (first 3 chars of device name) and take MAX
+        async def compute_pv_reduction(start, end):
+            q = await db.execute(
+                select(
+                    func.substring(Device.name, text("1"), text("3")).label("station"),
+                    func.max(EnergyData.value).label("max_energy"),
+                ).select_from(EnergyData).join(
+                    Device, EnergyData.device_id == Device.id
+                ).where(
+                    and_(
+                        EnergyData.timestamp >= start,
+                        EnergyData.timestamp < end,
+                        EnergyData.data_type == "daily_energy",
+                        Device.device_type.in_(["pv_inverter", "sungrow_inverter"]),
+                    )
+                ).group_by(text("station"))
+            )
+            total_kwh = sum(row[1] or 0 for row in q.all())
+            # Carbon reduction (kg CO2) = generation (kWh) * grid emission factor
+            return round(total_kwh * grid_factor / 1000, 4)  # convert to tons
+
+        today_reduction = await compute_pv_reduction(today_start, now)
+        month_reduction = await compute_pv_reduction(month_start, now)
+        year_reduction = await compute_pv_reduction(year_start, now)
+
+        today = {"emission": 0, "reduction": today_reduction}
+        month = {"emission": 0, "reduction": month_reduction}
+        year = {"emission": 0, "reduction": year_reduction}
+
     # 各scope分布
     scope_q = await db.execute(
         select(CarbonEmission.scope, func.sum(CarbonEmission.emission))