feat: customisable 404 + gallery-not-found pages via CMS (#324)

The 404 catch-all and the "gallery not found" branches in GalleryPage
were hard-coded English strings on a default-themed background — the
one place where a white-labelled deployment leaked the PicPeak default
look. Pluggable now via the existing CMS Pages mechanism.

Backend:
- Seed two new default CMS pages: `not-found` and `gallery-not-found`,
  with sensible English/German copy admins can edit in /admin/cms.
- Add `cms_pages.logo_url` (nullable) for per-page logo override; online
  migration on existing deployments. Null falls back to the global
  branding logo.
- New per-page logo upload (POST /api/admin/cms/pages/:slug/logo) +
  clear endpoint (DELETE …/logo). Reuses the existing /uploads/logos
  storage location with a `cms-<slug>-` filename prefix.
- adminCMS PUT now accepts logo_url; publicCMS GET returns it.

Frontend:
- New <CMSContentBlock slug fallback> component renders the CMS page in
  the standard branded shell (logo precedence: page → branding → bundled
  default), with DOMPurified content and footer/legal links.
- App.tsx: `path="*"` catch-all routes through CMSContentBlock("not-found").
- GalleryPage: collapses the two "gallery not found" branches (invalid
  identifier + infoError archived/missing) into a single
  CMSContentBlock("gallery-not-found"), so admins can edit one source
  of truth.
- Admin CMS Page editor gains an "Upload Logo / Use site default"
  control per page; falls back to the page's own English title in the
  page list when no `legal.<slug>` translation is registered.

frontend/src/services/cms.service.ts

@@ -7,6 +7,15 @@ export interface CMSPage {
   title_de: string;
   content_en: string;
   content_de: string;
+  logo_url: string | null;
+  updated_at: string;
+}
+
+export interface PublicCMSPage {
+  title: string;
+  content: string;
+  slug: string;
+  logo_url: string | null;
   updated_at: string;
 }
 
@@ -30,10 +39,27 @@ export const cmsService = {
   },
 
   // Get public CMS page (no auth required)
-  async getPublicPage(slug: string, lang: string = 'en'): Promise<{ title: string; content: string }> {
-    const response = await api.get<{ title: string; content: string }>(`/public/pages/${slug}`, {
+  async getPublicPage(slug: string, lang: string = 'en'): Promise<PublicCMSPage> {
+    const response = await api.get<PublicCMSPage>(`/public/pages/${slug}`, {
       params: { lang }
     });
     return response.data;
+  },
+
+  // Upload a per-page logo (#324)
+  async uploadPageLogo(slug: string, file: File): Promise<{ logo_url: string }> {
+    const formData = new FormData();
+    formData.append('logo', file);
+    const response = await api.post<{ logo_url: string }>(
+      `/admin/cms/pages/${slug}/logo`,
+      formData,
+      { headers: { 'Content-Type': 'multipart/form-data' } }
+    );
+    return response.data;
+  },
+
+  // Clear a per-page logo override (revert to global branding logo).
+  async clearPageLogo(slug: string): Promise<void> {
+    await api.delete(`/admin/cms/pages/${slug}/logo`);
   }
 };