refactor(insights): migrate route slugs to english with redirects

Rename every routed Insights sub-page slug from German to English
(`/insights/blutdruck` -> `/insights/blood-pressure`, `gewicht` ->
`weight`, `medikamente` -> `medications`, and the full v1.7.0 metric
tail). UI copy stays localised through `t()` keys; only the internal
slugs change. The slug registry in `src/lib/insights/sub-page-metric.ts`
is the single source of truth, so the tab strip, target summary, and
daily-briefing routing targets follow it.

`next.config.ts` `redirects()` emits a permanent redirect from every
legacy German slug to its English target, including a `:path*` variant
so nested routes survive. Bookmarks and cached PWA navigation keep
resolving.

src/app/api/insights/glp1-timeline/route.ts

@@ -4,7 +4,7 @@
  * Returns a chronological merge of every GLP-1-relevant event the user
  * has logged: dose changes, injections (with site if recorded),
  * inventory events, and side-effect-tagged mood entries from the
- * trailing 90 days. Used by /insights/medikamente's TherapyTimeline
+ * trailing 90 days. Used by /insights/medications' TherapyTimeline
  * component. Web-only users without a GLP-1 medication get
  * `hasGlp1: false` so the component hides cleanly.
  */