Expand direct image links instead of showing nothing (#43 follow-up)

A URL that points straight at an image file (Content-Type: image/*) has
no HTML to scrape Open Graph tags from, so the fetch found nothing and
the message showed no preview at all -- reported against
https://imgs.xkcd.com/comics/creepy.png.

link_preview_service now recognizes an allowed image content-type (same
list storage.py uses for uploads) before falling through to the HTML/og:
path, and returns the URL itself as the preview (LinkPreview.is_image).
No need to download the body -- the already-SSRF-validated URL is the
image. The frontend renders that case as a real expandable image
(message-image + lightbox, same as an actual attachment) instead of the
small title+description card, which would have nothing to show anyway.

Verified end-to-end against the reported URL.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/f3ec1c1d1992_link_previews_flag_direct_image_links.py

@@ -0,0 +1,35 @@
+"""link previews flag direct image links
+
+Revision ID: f3ec1c1d1992
+Revises: d00f93766fa5
+Create Date: 2026-08-17 17:56:19.078087
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'f3ec1c1d1992'
+down_revision: Union[str, Sequence[str], None] = 'd00f93766fa5'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # NOT NULL with a *constant* server_default (unlike a volatile one such
+    # as now()/clock_timestamp()) doesn't force a table rewrite in Postgres
+    # 11+ -- it's a metadata-only change, safe and instant regardless of
+    # table size.
+    op.add_column(
+        "link_previews",
+        sa.Column("is_image", sa.Boolean(), nullable=False, server_default=sa.false()),
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_column("link_previews", "is_image")