Meter chain topology + per-energy-type flow (Sankey) pages

Adds a meter hierarchy and a flow view: a downstream meter is a *subsection*
of an upstream one (not an addition), so you can see where a main meter's flow
divides — e.g. official water → garden, pool, other; grid/battery → all → car.

- MeterLink (schema + migration AddMeterLinks): a directed from→to flow edge.
  Multi-parent allowed (a merge, e.g. grid + solar → house); multi-child is a
  split. Cascade-deletes with either endpoint; unique + distinct-endpoint checks.
- FlowService: per energy type + period, builds a Sankey graph — nodes = meters
  sized by consumption; link value = downstream meter's consumption, split
  proportionally across multiple upstreams; unaccounted remainder under a meter
  becomes a synthetic "Other" node; depth via topological longest-path.
- SankeyChart.razor: hand-rolled inline-SVG Sankey (ApexCharts has no Sankey
  type) — columns by depth, nodes stacked by value, bezier ribbons sized by flow,
  left→right, theme-aware, HTML-encoded labels, tooltips. Built as a MarkupString
  to sidestep Razor's <text> element clash.
- /energy/{id} page (one per energy type): KPIs (consumption + cost), the flow
  Sankey, and the meter list. NavMenu now lists a link per energy type
  (Electricity, Water, Gas, …) loaded from the DB.
- Meters admin: cycle-safe "Sub-meter of (upstream meters)" multi-select
  (descendants excluded to prevent cycles); reconciles meter_link rows on save.
- Reference data seeds a demo chain (Haus → Auto) so electricity flow shows
  Haus dividing into Auto + Other.

Tests: FlowServiceTests (single-parent remainder; two-parent proportional
split); render test now asserts the flow chain + covers /energy/{id}. 69 Core +
47 Integration = 116 green. Live-verified: Haus 95,450 kWh → Auto 51,909 +
Other 43,541 (flow conserved), all 5 energy-type pages render.

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

tests/Integration.Tests/DashboardRenderTests.cs

@@ -46,6 +46,7 @@ public sealed class DashboardRenderTests(TimescaleFixture fx)
 
             // Panel read models compute real figures from the reference data (SDD §8.4–§8.6).
             int hausId;
+            short electricityTypeId;
             using (var scope = factory.Services.CreateScope())
             {
                 var services = scope.ServiceProvider;
@@ -72,6 +73,13 @@ public sealed class DashboardRenderTests(TimescaleFixture fx)
                 Assert.NotNull(detail);
                 Assert.True(detail!.ReadingCount > 0);
                 Assert.True(detail.TotalConsumption > 0);
+
+                // Flow graph: the demo Haus → Auto chain yields a link + an "Other (Haus)" remainder.
+                electricityTypeId = await db.EnergyTypes.Where(t => t.Key == "electricity").Select(t => t.Id).FirstAsync();
+                var flow = await services.GetRequiredService<FlowService>()
+                    .GetFlowAsync(electricityTypeId, new DateOnly(1997, 1, 1), new DateOnly(2027, 1, 1));
+                Assert.True(flow.HasChain);
+                Assert.Contains(flow.Nodes, n => n.IsOther);
             }
 
             using var client = factory.CreateClient();
@@ -91,6 +99,7 @@ public sealed class DashboardRenderTests(TimescaleFixture fx)
                 "/meters", "/trends", "/solar", "/consumables", "/import",
                 "/admin/tariffs", "/admin/energy-types", "/admin/categories",
                 "/admin/connectors", "/admin/settings", $"/meters/{hausId}",
+                $"/energy/{electricityTypeId}",
             })
             {
                 var response = await client.GetAsync(new Uri(path, UriKind.Relative));
@@ -106,6 +115,7 @@ public sealed class DashboardRenderTests(TimescaleFixture fx)
 
     private static async Task ClearDataAsync(MeterVaultDbContext db)
     {
+        await db.MeterLinks.ExecuteDeleteAsync();
         await db.Consumption.ExecuteDeleteAsync();
         await db.Readings.ExecuteDeleteAsync();
         await db.MeterEvents.ExecuteDeleteAsync();