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/Infrastructure/Persistence/Migrations/20260714114901_AddMeterLinks.cs

@@ -0,0 +1,60 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
+
+#nullable disable
+
+namespace MeterVault.Infrastructure.Persistence.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddMeterLinks : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "meter_link",
+                columns: table => new
+                {
+                    id = table.Column<int>(type: "integer", nullable: false)
+                        .Annotation("Npgsql:ValueGenerationStrategy", NpgsqlValueGenerationStrategy.IdentityByDefaultColumn),
+                    from_meter_id = table.Column<int>(type: "integer", nullable: false),
+                    to_meter_id = table.Column<int>(type: "integer", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("pk_meter_link", x => x.id);
+                    table.CheckConstraint("ck_meter_link_distinct", "from_meter_id <> to_meter_id");
+                    table.ForeignKey(
+                        name: "fk_meter_link_meter_from_meter_id",
+                        column: x => x.from_meter_id,
+                        principalTable: "meter",
+                        principalColumn: "id",
+                        onDelete: ReferentialAction.Cascade);
+                    table.ForeignKey(
+                        name: "fk_meter_link_meter_to_meter_id",
+                        column: x => x.to_meter_id,
+                        principalTable: "meter",
+                        principalColumn: "id",
+                        onDelete: ReferentialAction.Cascade);
+                });
+
+            migrationBuilder.CreateIndex(
+                name: "ix_meter_link_from_meter_id_to_meter_id",
+                table: "meter_link",
+                columns: new[] { "from_meter_id", "to_meter_id" },
+                unique: true);
+
+            migrationBuilder.CreateIndex(
+                name: "ix_meter_link_to_meter_id",
+                table: "meter_link",
+                column: "to_meter_id");
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "meter_link");
+        }
+    }
+}