Security audit: fix IDOR, add rate limiting, HMAC auth, token hashing, XSS sanitization

Critical: Add ownership checks to all order routes (IDOR fix).
High: Redis rate limiting on auth endpoints, HMAC-signed internal
service calls replacing header-presence-only checks, nh3 HTML
sanitization on ghost_sync and product import, internal auth on
market API endpoints, SHA-256 hashed OAuth grant/code tokens.
Medium: SECRET_KEY production guard, AP signature enforcement,
is_admin param removal, cart_sid validation, SSRF protection on
remote actor fetch.

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

events/bp/data/routes.py

@@ -19,6 +19,9 @@ def register() -> Blueprint:
     async def _require_data_header():
         if not request.headers.get(DATA_HEADER):
             return jsonify({"error": "forbidden"}), 403
+        from shared.infrastructure.internal_auth import validate_internal_request
+        if not validate_internal_request():
+            return jsonify({"error": "forbidden"}), 403
 
     _handlers: dict[str, object] = {}
 
@@ -131,8 +134,9 @@ def register() -> Blueprint:
         period_start = datetime.fromisoformat(request.args.get("period_start", ""))
         period_end = datetime.fromisoformat(request.args.get("period_end", ""))
         user_id = request.args.get("user_id", type=int)
-        is_admin = request.args.get("is_admin", "false").lower() == "true"
         session_id = request.args.get("session_id")
+        # is_admin determined server-side, never from client params
+        is_admin = False
         entries = await services.calendar.visible_entries_for_period(
             g.s, calendar_id, period_start, period_end,
             user_id=user_id, is_admin=is_admin, session_id=session_id,