Flow: virtual "sum" meters that aggregate upstreams + demo chain

Adds the "sum meter" the user asked for: a meter that doesn't physically exist
but represents the sum of other meters in the flow view.

- FlowService: a Virtual-mode meter has no readings of its own; its flow value is
  the sum of its upstream meters, resolved in topological order (so "Summe Solar"
  = Solar 1 + Solar 2, and Grid + Summe Solar → House with the remainder as
  "Other" = export / battery / inverter losses).
- Meters editor: a hint when Mode = Virtual explaining the sum-meter behaviour.
- Reference data seeds the full demo chain: Solar 1 + Solar 2 → Summe Solar;
  Netz + Summe Solar → Haus → Auto + Other — so /energy/1 shows a multi-level
  Sankey out of the box.
- Fix: SankeyChart Unit was passed as the literal string "_graph.Unit" (missing
  @) so node labels read "_graph.Unit" instead of "kWh". Caught by screenshotting
  the live page.

Test: Virtual_sum_meter_aggregates_its_upstreams. 69 Core + 50 Integration =
119 green. Live-verified with a browser screenshot of the multi-level flow.

Claude-Session: https://claude.ai/code/session_01Lz2RqAsnQhetqWNoCDfexK

tests/Integration.Tests/FlowServiceTests.cs

@@ -104,6 +104,45 @@ public sealed class FlowServiceTests(TimescaleFixture fx)
         }
     }
 
+    [Fact]
+    public async Task Virtual_sum_meter_aggregates_its_upstreams()
+    {
+        await using var db = fx.CreateContext();
+        try
+        {
+            var type = await SeedTypeAsync(db, "flow_elec_d");
+            var solar1 = await AddMeterAsync(db, "Solar 1", type);
+            var solar2 = await AddMeterAsync(db, "Solar 2", type);
+            var sumSolar = await AddMeterAsync(db, "Sum Solar", type, MeterMode.Virtual);
+            var grid = await AddMeterAsync(db, "Grid", type);
+            var house = await AddMeterAsync(db, "House", type);
+            // Solar1 + Solar2 → Sum Solar ; Grid + Sum Solar → House.
+            db.MeterLinks.Add(new MeterLink { FromMeterId = solar1.Id, ToMeterId = sumSolar.Id });
+            db.MeterLinks.Add(new MeterLink { FromMeterId = solar2.Id, ToMeterId = sumSolar.Id });
+            db.MeterLinks.Add(new MeterLink { FromMeterId = grid.Id, ToMeterId = house.Id });
+            db.MeterLinks.Add(new MeterLink { FromMeterId = sumSolar.Id, ToMeterId = house.Id });
+            await db.SaveChangesAsync();
+
+            await AddConsumptionAsync(db, solar1.Id, 15, ConsumptionKind.Generation);
+            await AddConsumptionAsync(db, solar2.Id, 15, ConsumptionKind.Generation);
+            await AddConsumptionAsync(db, grid.Id, 75);
+            await AddConsumptionAsync(db, house.Id, 40);
+
+            var graph = await new FlowService(fx).GetFlowAsync(type, new DateOnly(2024, 1, 1), new DateOnly(2024, 12, 31));
+
+            // Sum Solar has no readings but equals Solar 1 + Solar 2 = 30.
+            Assert.Equal(30, graph.Nodes.Single(n => n.MeterId == sumSolar.Id).Value, 1);
+            // House (40) splits across Grid (75) and Sum Solar (30) → 40*30/105 from solar.
+            Assert.Equal(40.0 * 30 / 105, graph.Links.Single(l => l.From == $"m{sumSolar.Id}" && l.To == $"m{house.Id}").Value, 1);
+            // No spurious remainder under Solar 1/2 (their whole output flows into Sum Solar).
+            Assert.DoesNotContain(graph.Nodes, n => n.IsOther && n.Id == $"other{solar1.Id}");
+        }
+        finally
+        {
+            await ClearAsync(db);
+        }
+    }
+
     private static async Task<short> SeedTypeAsync(MeterVaultDbContext db, string key)
     {
         var type = new EnergyType { Key = key, DisplayName = key, BaseUnit = "kWh", DefaultMode = MeterMode.CumulativeCounter };
@@ -112,9 +151,9 @@ public sealed class FlowServiceTests(TimescaleFixture fx)
         return type.Id;
     }
 
-    private static async Task<Meter> AddMeterAsync(MeterVaultDbContext db, string name, short type)
+    private static async Task<Meter> AddMeterAsync(MeterVaultDbContext db, string name, short type, MeterMode mode = MeterMode.DirectDelta)
     {
-        var meter = new Meter { Name = name, EnergyTypeId = type, Mode = MeterMode.DirectDelta, Unit = "kWh" };
+        var meter = new Meter { Name = name, EnergyTypeId = type, Mode = mode, Unit = "kWh" };
         db.Meters.Add(meter);
         await db.SaveChangesAsync();
         return meter;