feat(downloads): a clip or an edit can be a GIF, shared as a picture

A clip and an editor range can come out as a silent GIF (long side
320/480/640, 12 fps, at most 30 s, one palette per GIF); one over 30 MB
is not kept. A shared picture is served inline under the preview's
sandbox, and a GIF's link card is the GIF itself up to 15 MB, the still
for Facebook's crawler.

backend/app/routes/downloads.py

@@ -318,7 +318,9 @@ def enqueue_edit(
     except quota.QuotaExceeded as e:
         raise HTTPException(status_code=422, detail=quota_detail(e)) from e
     except service.EditError as e:
-        raise HTTPException(status_code=400, detail={"code": "edit", "reason": e.reason}) from e
+        raise HTTPException(
+            status_code=400, detail={**e.params, "code": "edit", "reason": e.reason}
+        ) from e
     db.commit()
     return serialize_job(db, job)
 
@@ -341,6 +343,8 @@ class EnqueueClipIn(BaseModel):
     audio_track: int | None = None
     # A crop frame (downloads/clip): fractions of the box it was drawn in, and that box's aspect.
     crop: ClipCropIn | None = None
+    # A GIF of this long side (downloads/gif) instead of a video; None is the video clip.
+    gif_side: int | None = None
     display_name: str | None = None
 
 
@@ -353,7 +357,7 @@ def enqueue_clip(
     try:
         crop = body.crop.model_dump() if body.crop else None
         spec = clipmod.normalize(
-            body.source, body.ref, body.start_s, body.end_s, body.audio_track, crop
+            body.source, body.ref, body.start_s, body.end_s, body.audio_track, crop, body.gif_side
         )
         display_name = display_text(body.display_name)
         job = service.enqueue_clip(db, user.id, spec, display_name=display_name)

backend/app/routes/files.py

@@ -715,13 +715,8 @@ def get_file(
     return response
 
 
-# A preview's own headers. `sandbox` leaves the answer no script and an opaque origin, whatever the
-# file turns out to hold; `frame-ancestors 'self'` lets the app's viewer frame a PDF (every other
-# answer keeps the app-wide 'none', main.py), with X-Frame-Options saying the same for old engines.
-PREVIEW_HEADERS = {
-    "Content-Security-Policy": "sandbox; frame-ancestors 'self'",
-    "X-Frame-Options": "SAMEORIGIN",
-}
+# A preview's own headers, shared with a share link's picture (`preview.SANDBOX_HEADERS`).
+PREVIEW_HEADERS = preview.SANDBOX_HEADERS
 # Set on a text preview cut at `preview.TEXT_PREVIEW_BYTES`, so the viewer can say it shows the start.
 TRUNCATED_HEADER = "X-Preview-Truncated"
 

backend/app/routes/public.py

@@ -16,7 +16,7 @@ from sqlalchemy.orm import Session
 from app.config import settings
 from app.db import get_db
 from app.downloads import links as linksmod
-from app.downloads import queries, storage
+from app.downloads import preview, queries, storage
 from app.downloads.status import JobStatus
 from app.ratelimit import RateLimiter, client_key
 from app.security import verify_password
@@ -226,8 +226,9 @@ def _granted(link: "DownloadLink", token: str, g: str | None) -> None:
 
 
 def _serve(link: "DownloadLink", asset: "MediaAsset") -> FileResponse:
-    """Range-aware stream of a shared file. Inline by default (plays in the page); an
-    `allow_download` link serves it as an attachment."""
+    """Range-aware stream of a shared file. Inline by default (a video plays and a picture shows in
+    the page, and a chat's link preview can fetch the picture); an `allow_download` link serves it
+    as an attachment."""
     path = storage.safe_abs_path(settings.files_root, asset.rel_path)
     if path is None:
         raise refused(404, "video_gone")
@@ -241,6 +242,17 @@ def _serve(link: "DownloadLink", asset: "MediaAsset") -> FileResponse:
     # served with its real audio/video type, inline unless the link forces a download so it can play
     # in the watch page. Both are range-aware (FileResponse → 206) for seeking/resume.
     media_type = storage.served_media_type(asset.media_kind, asset.container)
+    disposition = "attachment" if link.allow_download else "inline"
+    if media_type is None and preview.kind(asset.media_kind, asset.container) == "image":
+        # A picture (a GIF made from a clip, an uploaded photo) is shown as itself: its type from the
+        # preview allow-list, under the preview's sandbox, so a mislabelled one still runs nothing.
+        return FileResponse(
+            path,
+            filename=filename,
+            media_type=preview.media_type("image", asset.container),
+            content_disposition_type=disposition,
+            headers=preview.SANDBOX_HEADERS,
+        )
     if media_type is None:
         return FileResponse(
             path,
@@ -248,7 +260,6 @@ def _serve(link: "DownloadLink", asset: "MediaAsset") -> FileResponse:
             media_type="application/octet-stream",
             content_disposition_type="attachment",
         )
-    disposition = "attachment" if link.allow_download else "inline"
     return FileResponse(
         path, filename=filename, media_type=media_type, content_disposition_type=disposition
     )