Adds POST /api/posts/{post}/media for direct file (multipart) upload
and renames the existing URL-based flow to /api/posts/{post}/media/from-url
so the path matches HTTP semantics (POST <resource>/media expects a file
body, not JSON URLs).
The multipart action validates type against the post's enabled platforms
(image rejected on TikTok-only posts), enforces per-type size caps, and
reuses Workspace::addMedia + Post::appendMedia. URL-based attaching is
unchanged behaviorally — only the route name and controller method are
renamed for symmetry. The MCP AttachMediaFromUrlTool was already named
correctly and needs no changes; binary upload via MCP is a host-protocol
limitation that no MCP server (including Postiz) supports.
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