fix(gps-tracks): rename health fields and fix layer insert priority (F-04, F-05)

F-04: rename gps_health() response fields per tester feedback:
  - total_tracks → tracks_total
  - by_activity  → tracks_by_activity
  - recent_pipeline_runs (list) → last_pipeline_run (object | null)
  Change LIMIT from 10 to 1; fetch single row instead of a list.

F-05: rewrite _findGpsInsertPosition with explicit priority order:
  1. gpx-layer-* (ET-006 GPX file layers) — highest priority
  2. route-* (ET-002 routing layers)
  Remove old combined find() that lacked clear priority semantics.

Add tests/web/gps_tracks.test.js (22 JS unit tests via node:test):
  - _findGpsInsertPosition priority logic (9 cases)
  - Filter state management — default state assertions (5 cases)
  - Color palette mapping and _buildColorExpression (8 cases)

Add tests/web/test_gps_tracks.py — Python pytest runner (8 static
  checks + node --test invocation).

Refs: ET-008

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/web/gps_tracks.js

@@ -189,14 +189,25 @@ function _ensureGpsLayers(map) {
 }
 
 function _findGpsInsertPosition(map) {
+  /**
+   * Returns the id of the first layer that GPS tracks should be inserted
+   * below, using priority order:
+   *   1. gpx-layer-*  — ET-006 GPX file layers (highest priority)
+   *   2. route-*      — ET-002 routing layers
+   * Returns undefined if neither is present (GPS tracks go on top).
+   */
   const style = map.getStyle && map.getStyle();
   if (!style || !style.layers) return undefined;
-  const routeLayer = style.layers.find(l =>
-    l.id === 'route-line' ||
-    l.id.startsWith('route-') ||
-    l.id.startsWith('gpx-layer-')
-  );
-  return routeLayer ? routeLayer.id : undefined;
+
+  // Priority 1: gpx-layer-* (ET-006 GPX file layers)
+  const gpxLayer = style.layers.find(l => l.id.startsWith('gpx-layer-'));
+  if (gpxLayer) return gpxLayer.id;
+
+  // Priority 2: route-* (ET-002 routing layers)
+  const routeLayer = style.layers.find(l => l.id.startsWith('route-'));
+  if (routeLayer) return routeLayer.id;
+
+  return undefined;
 }
 
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