Fix remaining 'Find Link' references and improve error messages

- Rename index page title and heading to 'Missing Link'
- Give a clear message for 429 rate limit errors
- Include response body snippet in other unexpected API errors
- Log warning when falling back to unauthenticated session

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

add_links/api.py

@@ -81,6 +81,7 @@ def _get_active_session() -> requests.sessions.Session:
             return g.oauth_session  # type: ignore[return-value]
     except RuntimeError:
         pass
+    print("WARNING: using unauthenticated session", file=sys.stderr)
     return get_session()
 
 
@@ -96,7 +97,9 @@ def api_get(params: StrDict) -> StrDict:
         print(f"Response body: {r.text!r}", file=sys.stderr)
         if webpage_error in r.text:
             raise MediawikiError(webpage_error)
-        raise MediawikiError(f"HTTP {r.status_code}: unexpected response from Wikipedia API")
+        if r.status_code == 429:
+            raise MediawikiError("Wikipedia rate limit exceeded — wait a moment and try again.")
+        raise MediawikiError(f"HTTP {r.status_code}: {r.text[:200]!r}")
     check_for_error(ret)
     return ret