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

src/App/Components/Pages/Meters.razor

@@ -87,6 +87,15 @@ else
             <MudSelectItem T="string" Value="@MeterRoles.GridImport">grid_import</MudSelectItem>
             <MudSelectItem T="string" Value="@MeterRoles.GridExport">grid_export</MudSelectItem>
         </MudSelect>
+        <MudSelect T="int" MultiSelection="true" @bind-SelectedValues="_working.Upstream"
+                   Label="Sub-meter of (upstream meters)" Class="mb-2"
+                   MultiSelectionTextFunc="@(ids => UpstreamText(ids))"
+                   HelperText="This meter measures a subsection of the selected meter(s)' flow.">
+            @foreach (var m in AvailableUpstream())
+            {
+                <MudSelectItem T="int" Value="m.Id">@m.Name</MudSelectItem>
+            }
+        </MudSelect>
         <MudTextField @bind-Value="_working.Location" Label="Location (optional)" Class="mb-2" />
         <MudTextField @bind-Value="_working.SerialNumber" Label="Serial number (optional)" Class="mb-2" />
         <div class="d-flex" style="gap:1rem">
@@ -108,6 +117,7 @@ else
 @code {
     private List<Meter>? _meters;
     private List<EnergyType> _energyTypes = [];
+    private List<MeterLink> _allLinks = [];
     private bool _editOpen;
     private EditModel _working = new();
     private readonly DialogOptions _dialogOptions = new() { MaxWidth = MaxWidth.Small, FullWidth = true };
@@ -118,6 +128,7 @@ else
     {
         await using var db = await DbFactory.CreateDbContextAsync();
         _energyTypes = await db.EnergyTypes.AsNoTracking().OrderBy(t => t.DisplayName).ToListAsync();
+        _allLinks = await db.MeterLinks.AsNoTracking().ToListAsync();
         _meters = await db.Meters
             .AsNoTracking()
             .Include(m => m.EnergyType)
@@ -126,6 +137,49 @@ else
             .ToListAsync();
     }
 
+    // Upstream candidates: same energy type, not self, and not a descendant (would create a cycle).
+    private IEnumerable<Meter> AvailableUpstream()
+    {
+        if (_meters is null)
+        {
+            return [];
+        }
+
+        var descendants = Descendants(_working.Id);
+        return _meters.Where(m => m.EnergyTypeId == _working.EnergyTypeId && m.Id != _working.Id && !descendants.Contains(m.Id));
+    }
+
+    private HashSet<int> Descendants(int meterId)
+    {
+        var result = new HashSet<int>();
+        if (meterId == 0)
+        {
+            return result;
+        }
+
+        var queue = new Queue<int>();
+        queue.Enqueue(meterId);
+        while (queue.Count > 0)
+        {
+            var current = queue.Dequeue();
+            foreach (var link in _allLinks.Where(l => l.FromMeterId == current))
+            {
+                if (result.Add(link.ToMeterId))
+                {
+                    queue.Enqueue(link.ToMeterId);
+                }
+            }
+        }
+
+        return result;
+    }
+
+    private string UpstreamText(IReadOnlyList<string> ids)
+    {
+        var names = ids.Select(idText => int.TryParse(idText, out var id) ? _meters?.FirstOrDefault(m => m.Id == id)?.Name ?? idText : idText);
+        return string.Join(", ", names);
+    }
+
     private void OpenEdit(Meter? meter)
     {
         if (meter is null)
@@ -150,6 +204,7 @@ else
                 Manufacturer = meter.Manufacturer,
                 Model = meter.Model,
                 IsActive = meter.IsActive,
+                Upstream = _allLinks.Where(l => l.ToMeterId == meter.Id).Select(l => l.FromMeterId).ToHashSet(),
             };
         }
         _editOpen = true;
@@ -164,9 +219,10 @@ else
         }
 
         await using var db = await DbFactory.CreateDbContextAsync();
+        int meterId;
         if (_working.Id == 0)
         {
-            db.Meters.Add(new Meter
+            var meter = new Meter
             {
                 Name = _working.Name.Trim(),
                 EnergyTypeId = _working.EnergyTypeId,
@@ -179,8 +235,10 @@ else
                 Manufacturer = Trim(_working.Manufacturer),
                 Model = Trim(_working.Model),
                 IsActive = _working.IsActive,
-            });
+            };
+            db.Meters.Add(meter);
             await db.SaveChangesAsync();
+            meterId = meter.Id;
         }
         else
         {
@@ -208,13 +266,35 @@ else
             }
 
             await tx.CommitAsync();
+            meterId = existing.Id;
         }
 
+        await SyncUpstreamAsync(db, meterId, _working.Upstream);
+
         _editOpen = false;
         Snackbar.Add("Saved.", Severity.Success);
         await LoadAsync();
     }
 
+    /// <summary>Reconciles the meter's incoming flow links to the selected upstream meters.</summary>
+    private static async Task SyncUpstreamAsync(MeterVault.Infrastructure.Persistence.MeterVaultDbContext db, int meterId, IEnumerable<int> desiredUpstream)
+    {
+        var desired = desiredUpstream.Where(id => id != meterId).ToHashSet();
+        var existing = await db.MeterLinks.Where(l => l.ToMeterId == meterId).ToListAsync();
+
+        foreach (var link in existing.Where(l => !desired.Contains(l.FromMeterId)))
+        {
+            db.MeterLinks.Remove(link);
+        }
+
+        foreach (var fromId in desired.Where(id => existing.All(l => l.FromMeterId != id)))
+        {
+            db.MeterLinks.Add(new MeterLink { FromMeterId = fromId, ToMeterId = meterId });
+        }
+
+        await db.SaveChangesAsync();
+    }
+
     private async Task DeleteAsync(Meter meter)
     {
         await using var db = await DbFactory.CreateDbContextAsync();
@@ -258,6 +338,7 @@ else
         public string? Manufacturer { get; set; }
         public string? Model { get; set; }
         public bool IsActive { get; set; } = true;
+        public IReadOnlyCollection<int> Upstream { get; set; } = new HashSet<int>();
 
         public bool RecomputeNeeded => Mode != OriginalMode || Math.Abs(InitialBaseline - OriginalBaseline) > 1e-9;
     }