feat(media): name video posters by hash in imeta

desktop/src/features/messages/lib/imetaMediaMarkdown.test.mjs

@@ -181,6 +181,22 @@ test("buildImetaTags keeps media filenames in imeta", () => {
   );
 });
 
+test("buildImetaTags names a video poster by hash", () => {
+  const poster = "c".repeat(64);
+  const [tag] = buildImetaTags([
+    {
+      url: `https://b/media/${"a".repeat(64)}.mp4`,
+      type: "video/mp4",
+      sha256: "a".repeat(64),
+      size: 10,
+      uploaded: 1,
+      image: `https://b/media/${poster}.jpg`,
+    },
+  ]);
+  assert.ok(tag.includes(`image https://b/media/${poster}.jpg`));
+  assert.ok(tag.includes(`image-x ${poster}`));
+});
+
 test("formatImetaMediaLine: video mime → ![video] line (regardless of URL suffix)", () => {
   assert.equal(
     formatImetaMediaLine({ url: "https://cdn/blob/xyz", type: "video/mp4" }),

desktop/src/features/messages/lib/imetaMediaMarkdown.ts

@@ -102,10 +102,19 @@ export function buildImetaTags(
       ...(d.thumb ? [`thumb ${d.thumb}`] : []),
       ...(d.duration != null ? [`duration ${d.duration}`] : []),
       ...(d.image ? [`image ${d.image}`] : []),
+      ...posterHashEntry(d.image),
       ...(d.filename ? [`filename ${d.filename}`] : []),
     ]);
 }
 
+const MEDIA_HASH_RE = /\/media\/([0-9a-f]{64})\./;
+
+/** `image-x <sha256>` names the poster by hash so the relay can scope its reads. */
+function posterHashEntry(image: string | undefined): string[] {
+  const hash = image?.match(MEDIA_HASH_RE)?.[1];
+  return hash ? [`image-x ${hash}`] : [];
+}
+
 const MEDIA_LINE_RE =
   /^(?:\|\|)?!\[(?:image|video)\]\(([^)\s]+)\)(?:\|\|)?\s*$/;
 const SPOILERED_MEDIA_LINE_RE =

mobile/lib/shared/relay/media_upload.dart

@@ -218,9 +218,14 @@ class BlobDescriptor {
     if (thumb != null) 'thumb $thumb',
     if (duration != null) 'duration $duration',
     if (image != null) 'image $image',
+    if (_posterHash case final hash?) 'image-x $hash',
     if (filename != null) 'filename $filename',
   ];
 
+  /// Names the poster by hash so the relay can scope who may read it.
+  String? get _posterHash =>
+      RegExp(r'/media/([0-9a-f]{64})\.').firstMatch(image ?? '')?.group(1);
+
   String toMarkdownImage() {
     final label = (filename ?? 'file').replaceAllMapped(
       RegExp(r'[\\\[\]]'),

mobile/test/shared/relay/media_upload_test.dart

@@ -1222,6 +1222,22 @@ void main() {
     });
   });
 
+  group('toImetaTag', () {
+    test('names a video poster by hash', () {
+      final poster = 'c' * 64;
+      final descriptor = BlobDescriptor(
+        url: 'https://relay.example/media/${'a' * 64}.mp4',
+        sha256: 'a' * 64,
+        size: 3,
+        type: 'video/mp4',
+        uploaded: 1,
+        image: 'https://relay.example/media/$poster.jpg',
+      );
+
+      expect(descriptor.toImetaTag(), contains('image-x $poster'));
+    });
+  });
+
   group('uploadVoiceNote', () {
     test('keeps ordinary audio attachments on inline audio markdown', () {
       const descriptor = BlobDescriptor(