fix(i18n): German translations for insights.recommendation.{source,viewSource}

Two keys were added to en.json in `a66c128` (B5a citation footnote)
but never mirrored in de.json, so the locale-parity guard test went
red on every push and German users saw the raw key name in place of
the citation label. This restores parity by adding "Quelle" + the
"in neuem Tab öffnen" tooltip without touching feature scope.

The component appends the colon at render time, so the value is
"Quelle" rather than "Quelle:" — the locale-integrity test plus the
existing citation-footnote rendering test both go green.

messages/de.json

@@ -665,6 +665,10 @@
     "heroFindingWarning": "Wichtigster Alarm",
     "correlationsTitle": "Zusammenhänge",
     "recommendationsTitle": "Empfehlungen",
+    "recommendation": {
+      "source": "Quelle",
+      "viewSource": "Leitlinie in neuem Tab öffnen"
+    },
     "dataFoundation": "Datengrundlage",
     "dataGaps": "Datenlücken:",
     "lastUpdated": "Zuletzt aktualisiert",