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>

shared/sx/handlers.py

@@ -24,11 +24,25 @@ import logging
 import os
 from typing import Any, Callable, Awaitable
 
+from werkzeug.routing import BaseConverter
+
 from .types import HandlerDef
 
 logger = logging.getLogger("sx.handlers")
 
 
+class SxAtomConverter(BaseConverter):
+    """URL converter for SX atoms inside expression URLs.
+
+    Matches a single atom — stops at dots, parens, slashes, and query chars.
+    Use as ``<sx:param>`` in route patterns like::
+
+        /(geography.(hypermedia.(reference.(api.(item.<sx:item_id>)))))
+    """
+
+    regex = r"[^./)(?&= ]+"
+
+
 # ---------------------------------------------------------------------------
 # Registry — service → handler-name → HandlerDef
 # ---------------------------------------------------------------------------
@@ -229,6 +243,10 @@ def register_route_handlers(app_or_bp: Any, service_name: str) -> int:
     from quart import Response, request
     from shared.browser.app.csrf import csrf_exempt
 
+    # Register SX atom converter for expression URL parameters
+    if hasattr(app_or_bp, 'url_map'):
+        app_or_bp.url_map.converters.setdefault('sx', SxAtomConverter)
+
     handlers = get_all_handlers(service_name)
     count = 0