feat(routing): linked complexity-boundary sliders — your own low/medium/high

The tier boundaries lived on the router and were global for every customer
of a hosted router; per-gateway tuning meant hand-editing score ranges. The
Routing page now has one track with two draggable handles (the low|medium
and medium|high boundaries) over three colored zones: handles push each
other (low can't cross medium — it shoves it along, and vice versa), the
numbers ride on the handles and the legend updates live, so nobody types
ranges by hand. Unchecked = the router's own bands, untouched behavior.

Under the hood the gateway re-maps the router's CONTINUOUS complexity score
with the user's boundaries ([routing.bands] low/medium in gateway.toml,
validated 0 < low < medium < 1) before tier chains, escalation and stats —
verified live: with bands at 0.70/0.72 a score-0.27 request that used to be
medium routed low, and back after restoring the defaults.

2 new strings in all 7 languages.

web/js/i18n/de.js

@@ -629,4 +629,6 @@ export default {
   "routing.tasks.tierHint": "Welches Modell diese Aufgabe bei welcher Komplexität bedient; \"jede\" führt auf allen Stufen. Reihenfolge = Fallback-Reihenfolge. High eskaliert weiterhin über \"jede\"-Einträge.",
   "routing.tasks.tierAny": "jede Komplexität",
   "routing.tasks.addModel": "Modell",
+  "routing.bands.enable": "Eigene Komplexitätsgrenzen",
+  "routing.bands.hint": "Ziehen Sie die Griffe: wo low endet und high beginnt — die Griffe schieben einander, die Zahlen aktualisieren sich live. Der kontinuierliche Router-Score wird mit IHREN Grenzen neu eingeteilt; ohne Häkchen gelten die Grenzen des Routers.",
 };