Convert all API endpoint URLs to SX expression format

Every URL at sx-web.org now uses bracketed SX expressions — pages AND
API endpoints. defhandler :path values, sx-get/sx-post/sx-delete attrs,
code examples, and Python route decorators all converted.

- Add SxAtomConverter to handlers.py for parameter matching inside
  expression URLs (e.g. /(api.(item.<sx:item_id>)))
- Convert ~50 defhandler :path values in ref-api.sx and examples.sx
- Convert ~90 sx-get/sx-post/sx-delete URLs in reference.sx, examples.sx
- Convert ~30 code example URLs in examples-content.sx
- Convert ~30 API URLs in pages.py (Python string code examples)
- Convert ~70 page navigation URLs in pages.py
- Convert 7 Python route decorators in routes.py
- Convert ~10 reactive API URLs in marshes.sx
- Add API redirect patterns to sx_router.py (301 for old paths)
- Remove /api/ skip in app.py redirects (old API paths now redirect)

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

sx/app.py

@@ -128,9 +128,6 @@ def create_app() -> "Quart":
         # Skip SX expression URLs (already in new format)
         if path.startswith("/(") or path.startswith("/~"):
             return None
-        # Skip API/handler paths
-        if "/api/" in path:
-            return None
         new_url = redirect_old_url(path)
         if new_url:
             qs = request.query_string.decode()
@@ -149,7 +146,6 @@ def create_app() -> "Quart":
                 and not path.endswith("/")
                 and request.method == "GET"
                 and not path.startswith(("/static/", "/internal/", "/auth/"))
-                and "/api/" not in path
                 and "." not in path.rsplit("/", 1)[-1]):
             qs = request.query_string.decode()
             target = path + "/" + ("?" + qs if qs else "")