feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

frontend/src/lib/queryKeys.ts

@@ -68,6 +68,7 @@ export const qk = {
   // Every listing's key starts with "files", so one invalidation reaches each folder, search and sort.
   files: (p?: FileListParams) => (p ? (["files", p] as const) : (["files"] as const)),
   fileFolders: () => ["file-folders"] as const,
+  filePreview: (id: number) => ["file-preview", id] as const,
   storageUsage: () => ["storage-usage"] as const,
   // --- downloads ---
   downloads: () => ["downloads"] as const,