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/services/ssrf.py

@@ -3,27 +3,30 @@ import socket
 from urllib.parse import urlparse
 
 
-class UnsafeWebhookUrlError(Exception):
+class UnsafeUrlError(Exception):
     pass
 
 
 def validate_target_url(url: str) -> None:
-    """Creation-time-only SSRF check: rejects non-http(s) schemes and any
-    target whose hostname resolves to a private/loopback/link-local/
-    reserved/multicast address. Not re-checked per delivery, so this doesn't
-    defend against DNS rebinding between creation and a later send -- a
-    documented known limitation, not an oversight.
+    """One-shot SSRF check: rejects non-http(s) schemes and any target whose
+    hostname resolves to a private/loopback/link-local/reserved/multicast
+    address. Shared by two callers with different re-check needs: webhook
+    subscriptions validate once at creation time and reuse the URL for many
+    future deliveries (a real but accepted DNS-rebinding gap, documented
+    here), while link_preview_service calls this fresh before *every* hop of
+    a redirect chain for a one-shot fetch, which closes that gap for its own
+    use case.
     """
     parsed = urlparse(url)
     if parsed.scheme not in ("http", "https"):
-        raise UnsafeWebhookUrlError()
+        raise UnsafeUrlError()
     if not parsed.hostname:
-        raise UnsafeWebhookUrlError()
+        raise UnsafeUrlError()
 
     try:
         addrinfo = socket.getaddrinfo(parsed.hostname, None)
     except socket.gaierror as exc:
-        raise UnsafeWebhookUrlError() from exc
+        raise UnsafeUrlError() from exc
 
     for *_rest, sockaddr in addrinfo:
         ip = ipaddress.ip_address(sockaddr[0])
@@ -35,4 +38,4 @@ def validate_target_url(url: str) -> None:
             or ip.is_multicast
             or ip.is_unspecified
         ):
-            raise UnsafeWebhookUrlError()
+            raise UnsafeUrlError()