Add URL previews for chat messages (#43)

Slack/Discord-style link unfurling: the first http(s) URL in a message's
content gets a small preview card (title/description/image/site name)
fetched from the page's Open Graph tags.

Backend:
- Message.preview_url (extracted at create/edit time, cheap regex, no
  I/O) points at a link_previews cache row keyed by URL -- the same URL
  posted in different messages/rooms fetches once, and a failed fetch is
  cached too so a dead URL isn't retried on every reference.
- The actual fetch runs in a background asyncio.create_task from
  broadcast_new_message/broadcast_message_update, on its own DB session,
  so a slow third-party site never delays message delivery. A separate
  "link_preview" WS envelope carries the result once it resolves.
- SSRF protection reuses app/services/ssrf.py's validate_target_url
  (renamed from UnsafeWebhookUrlError to UnsafeUrlError now that it's
  shared with webhooks), but re-validates before every hop of a redirect
  chain rather than once up front -- redirects are followed manually so
  each intermediate URL is checked before it's ever connected to.
- Parsed with stdlib html.parser -- no new dependency.

Frontend: a LinkPreviewCard rendered under message content when present,
patched into state live via the new WS envelope and included in message
history for reloads.

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

backend/app/models/__init__.py

@@ -4,6 +4,7 @@ from app.models.base import Base
 from app.models.custom_theme import CustomTheme
 from app.models.event_subscription import EventSubscription
 from app.models.invite import InviteStatus
+from app.models.link_preview import LinkPreview
 from app.models.membership import RoomMembership, RoomRole
 from app.models.message import Message
 from app.models.message_file import MessageFile
@@ -41,4 +42,5 @@ __all__ = [
     "WebhookIncoming",
     "EventSubscription",
     "CustomTheme",
+    "LinkPreview",
 ]