feat(admin): add media library with uploads and reusable picker

[mighty840]

Summary

• Adds a Media Library page at /design/media with drag-and-drop upload, multi-select uploads, copy-URL action, and delete.
• Files are stored locally under /uploads (already served by the server) and indexed in a new MediaAsset table that captures filename, original name, MIME type, size, and uploader.
• Ships a reusable MediaPickerModal component so other admin forms (gallery, hero, menu items, banners) can adopt it incrementally without re-implementing the listing/upload flow.

Covers item 3 of the Moksha post-pitch backlog (image upload + media library).

New endpoints (staff-only):

• GET /api/media — paginated list
• POST /api/media/upload — multipart upload (5 MB, image only)
• DELETE /api/media/:id — removes DB row + underlying file

Test plan

• Navigate to Design → Media Library in admin
• Drag an image onto the drop zone — appears in the grid after upload
• Click "Upload Image" + select multiple files — all upload
• Click "Copy URL" — full URL copied, button shows "Copied!" feedback
• Delete an image — confirms via dialog, then disappears from grid and uploads/ folder
• Upload an oversized (>5 MB) or non-image file — gracefully fails with an error message

Follow-ups

• Wire MediaPickerModal into the gallery admin form (F2's DesignGallery) to replace the manual URL paste with a "Pick from library" button.
• Same for menu item images, hero background, branding logo, etc.

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