Media: uploads stripped of metadata, attachments both ways, a remote media proxy

- /api/v1/media and /api/v2/media (and GET/PUT /api/v1/media/:id):
  images go through libvips (NetVips, its native build bundled):
  autorotated, every kind of metadata dropped (EXIF, GPS, XMP, IPTC,
  comments), capped at 4096 px, with a 640 px preview and a blurhash
  (own encoder, the reference algorithm); animated GIFs are re-encoded;
  video and audio are remuxed by ffmpeg with -map_metadata -1, never
  re-encoded, and a video gets a still preview. Files get random names
  under /var/lib/privapub/media, outside the web root deploys replace, and
  are served at /media/files with nosniff and a sandbox CSP.
- media_ids on create and edit (four at most, the persona's own, each used
  once); notes carry them as Document attachments with alt text, blurhash,
  focalPoint and size; inbound attachments were already kept.
- avatar and header uploads in update_credentials, cropped to 400x400 and
  1500x500, federated with Update{Person}.
- Remote media reaches clients only through /media/proxy/{hmac}/{url},
  fetched by the guarded client (no SVG, 40 MB cap) and cached outside the
  served root, trimmed to 5 GB; foreign avatars and headers use it too, so
  a client never contacts another server.
- MediaJanitor deletes uploads left unattached for a day.
- nginx accepts 100 MB bodies on the upload endpoints only (applied on Max).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub/PrivaPub.csproj

@@ -7,11 +7,14 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="FFMpegCore" Version="5.5.0" />
     <PackageReference Include="HtmlSanitizer" Version="9.2.1039" />
     <PackageReference Include="MailKit" Version="4.18.0" />
     <PackageReference Include="Markdig" Version="1.4.0" />
     <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="10.0.9" />
     <PackageReference Include="MongoDB.Entities" Version="25.1.0" />
+    <PackageReference Include="NetVips" Version="3.2.0" />
+    <PackageReference Include="NetVips.Native.linux-x64" Version="8.18.7" />
     <PackageReference Include="OpenIddict.AspNetCore" Version="7.7.1" />
     <PackageReference Include="OpenIddict.MongoDb" Version="7.7.1" />
     <PackageReference Include="PasswordGenerator" Version="3.0.0" />