refs(wiki): shrink login background, undo the branding ConfigMap split

Downscale alpenglow.jpg 2560->1920px (604KB -> 400KB, visually identical on the
login screen). With the smaller background everything fits in one platform-branding
ConfigMap again (~752KB / 1MiB), so drop platform-branding-bg and its /_assets/img/bg
mount; authLoginBgUrl goes back to /_assets/img/branding/alpenglow.jpg. Favicons kept
as ThreadNet.

apps/production/kustomization.yaml

@@ -97,15 +97,10 @@ configMapGenerator:
     namespace: matrix
     files:
       - branding/logo.png
+      - branding/alpenglow.jpg
       - branding/favicon.ico
       - branding/favicons/favicon-32x32.png
       - branding/favicons/favicon-16x16.png
       - branding/favicons/android-chrome-192x192.png
       - branding/favicons/apple-touch-icon.png
       - branding/favicons/mstile-150x150.png
-  # Hintergrundbild separat: alpenglow.jpg (604 KB) würde die obige ConfigMap ans
-  # 1-MiB-Limit bringen. Eigene ConfigMap, eigener Mount-Pfad (/_assets/img/bg/).
-  - name: platform-branding-bg
-    namespace: matrix
-    files:
-      - branding/alpenglow.jpg

apps/production/wikijs-config.py

@@ -199,7 +199,7 @@ def ensure_theming(jwt: str):
     # wikijs.yaml + platform-branding ConfigMap): öffentlich ausgeliefert, kein read:assets
     # für Guests, keine externe URL. Dark als Default (#0050).
     logo = "/_assets/img/branding/logo.png"
-    bg = "/_assets/img/bg/alpenglow.jpg"
+    bg = "/_assets/img/branding/alpenglow.jpg"
     tc = gql('{theming{config{theme iconset darkMode tocPosition injectCSS injectHead injectBody}}}',
              jwt)["theming"]["config"]
     gql('mutation($t:String!,$i:String!,$d:Boolean!,$tp:String,$c:String,$h:String,$b:String){'

apps/production/wikijs.yaml

@@ -66,9 +66,6 @@ spec:
             - name: branding
               mountPath: /wiki/assets/img/branding
               readOnly: true
-            - name: branding-bg
-              mountPath: /wiki/assets/img/bg
-              readOnly: true
             # Favicon (#0050): serve-favicon liest /wiki/assets/favicon.ico; der
             # Browser-Tab nutzt zusätzlich die PNGs unter /_assets/favicons/. Einzelne
             # Dateien per subPath überlagern, ohne die restlichen Default-Favicons zu verdecken.
@@ -113,9 +110,6 @@ spec:
         - name: branding
           configMap:
             name: platform-branding
-        - name: branding-bg
-          configMap:
-            name: platform-branding-bg
 ---
 apiVersion: v1
 kind: Service