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/tests/test_webhooks.py

@@ -10,22 +10,22 @@ from app.models import PushSubscription
 from app.schemas.user import UserCreate
 from app.services.auth_service import register_user
 from app.services.room_service import join_room
-from app.services.ssrf import UnsafeWebhookUrlError, validate_target_url
+from app.services.ssrf import UnsafeUrlError, validate_target_url
 from tests.conftest import register_and_login
 
 
 def test_validate_target_url_rejects_loopback():
-    with pytest.raises(UnsafeWebhookUrlError):
+    with pytest.raises(UnsafeUrlError):
         validate_target_url("http://127.0.0.1/hook")
 
 
 def test_validate_target_url_rejects_private_range():
-    with pytest.raises(UnsafeWebhookUrlError):
+    with pytest.raises(UnsafeUrlError):
         validate_target_url("http://10.0.0.5/hook")
 
 
 def test_validate_target_url_rejects_non_http_scheme():
-    with pytest.raises(UnsafeWebhookUrlError):
+    with pytest.raises(UnsafeUrlError):
         validate_target_url("ftp://8.8.8.8/hook")