Node view: overlay child sensors on one time chart over a window (v1)

[lotgon]

Problem

Selecting a tree node shows each child sensor as its own separate tile/plot. To answer "which child was hottest over the last hour" — which process used the most CPU, which NIC moved the most data, which disk was busiest — the operator has to open each child sensor one by one and eyeball its chart. There is no node-level combined view.

(Dashboards already support multi-line panels, but you must add each sensor to a panel by hand. This is the zero-config, node-driven version.)

Proposal — v1 (this issue)

On a tree node, add a "Chart" tab that overlays every comparable child sensor's history as one multi-line time chart over the operator-chosen window (last hour / 3h / day / custom).

Scope for v1:

• Same-unit nodes only — all children share a sensor type + unit. Real examples on a live agent: Top CPU processes (all %), Network per-interface throughput (all MB/s). Mixed-unit nodes (e.g. Disks monitoring: %, MB/s, GB) are out of scope here (see v3).
• Read-only, derived entirely from stored history — never writes or changes sensor state.
• One line per child. Children with no data in the window are omitted (not zero-filled). Sparse series are drawn with gaps, not interpolated — top-N folders (Top CPU processes) report a process only while it is in the top-10 at ≥1%, so most lines are intermittent by design; a short UI note should explain this.
• No view when there are < 2 comparable children (fall back to the normal node panel) — e.g. a host with a single disk has nothing to overlay.

Implementation notes (verified anchors)

• Children of a node: TreeViewModel.GetAllNodeSensors(Guid) (already wrapped by HomeController.GetNodeSensors) returns all descendant sensor ids.
• History read: same path SensorHistoryController.ChartHistory uses (GetSensorHistoryRequest). Precedent for one request → many series is the Grafana datasource query (QueryHistoryRequest.Targets[]). Add a node-level batch endpoint (node id + window + optional unit-group) that fans out over GetAllNodeSensors and returns the full series set in one call.
• Client render: Plotly.js 2.28.0 is already bundled and used by Views/Home/Sensor/History/_SensorGraphTabContent.cshtml — reuse it for the overlay.
• UI surface: new tab in the node panel — HomeController.SelectNode → Views/Home/_NodeDataPanel.cshtml.

Out of scope (tracked separately, later)

• v2 — Ranking: fold the same series into a sorted "top consumers" leaderboard (total / average / peak) as a second tab on the same data.
• v3: mixed-unit nodes (one chart/group per unit), scope toggle (direct children vs flattened subtree), per-type aggregation strategies (rate/counter delta, enum/bool availability = % uptime / downtime / flaps).

Acceptance (v1)

• Select Top CPU processes → Chart tab → last hour → one chart with a line per process.
• Select Network → one line per interface (MB/s).
• Node with < 2 comparable children → no Chart tab.
• Selecting a period re-queries and redraws; children with no data in the window are absent, not zeroed.

Feature doc

aicontext/features/site/node-children-view/feature.md (companion docs PR) — full behavior, invariants, and the v1/v2/v3 progression.