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

README.md

@@ -26,9 +26,14 @@ full design.
   savings), an **oil/consumable panel** (tank gauge, deliveries, burner runtime, effective L/h,
   forecast-to-empty) and a **per-meter detail view** (raw readings, consumption, sources, tariff
   timeline, events), one-click reference-data load, CSV dry-run.
+- **Per-energy-type flow pages** (Electricity, Water, …): a **Sankey diagram** of the meter chain —
+  a downstream meter is a *subsection* of an upstream one (main → car, pool, garden, …), arrow
+  thickness ∝ amount, with an auto-computed "Other/unmetered" remainder. Meters can have several
+  upstreams (a merge, e.g. grid + solar → house).
 - **Admin UI**: full create/edit/delete for energy types, meters (with consumption recompute on
-  mode/baseline change), ingest sources, tariffs, cost categories, and MQTT/Home-Assistant
-  connectors; a "Test connection" for Home Assistant; effective-settings view.
+  mode/baseline change, and cycle-safe upstream-meter wiring), ingest sources, tariffs, cost
+  categories, and MQTT/Home-Assistant connectors; a "Test connection" for Home Assistant;
+  effective-settings view.
 - **REST API + OpenAPI/Swagger**, API-key auth, reverse-proxy trust (Authelia/Traefik).
 - **JSON config export/import** for portability; Docker Compose + multi-arch image.