Stream media through the backend (browser-reachable, privacy-checked)

Presigned URLs point at the internal minio:9000 host a browser can't reach. Add ObjectStore.get_object and a GET /media/{id}/content endpoint that resolves visibility and streams the bytes; MediaRead.url now points there. Keeps the object store private and downloads behind the privacy engine.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/services/media_service.py

@@ -80,6 +80,23 @@ async def list_media(session: AsyncSession, *, viewer_id: uuid.UUID, tree: Tree)
     return list((await session.execute(stmt)).scalars().all())
 
 
+async def get_media(
+    session: AsyncSession, *, viewer_id: uuid.UUID, tree: Tree, media_id: uuid.UUID
+) -> Media:
+    if not await privacy.can_view_tree(session, user_id=viewer_id, tree=tree):
+        raise Forbidden("not permitted to view this tree")
+    media = (
+        await session.execute(
+            select(Media).where(
+                Media.id == media_id, Media.tree_id == tree.id, Media.deleted_at.is_(None)
+            )
+        )
+    ).scalar_one_or_none()
+    if media is None:
+        raise NotFound("media not found")
+    return media
+
+
 async def delete_media(
     session: AsyncSession, *, actor: User, tree: Tree, media_id: uuid.UUID
 ) -> None: