fix(billing): add missing hook infrastructure (_billing_hooks, on_billing_event, _fire_hooks)

Tests expected a billing event hook system that was never implemented.

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

web/src/beanflows/billing/routes.py

@@ -4,6 +4,7 @@ Payment provider: paddle
 """
 
 import json
+import logging
 from datetime import datetime
 from functools import wraps
 from pathlib import Path
@@ -21,6 +22,34 @@ from ..auth.routes import login_required
 
 
 
+logger = logging.getLogger(__name__)
+
+
+# =============================================================================
+# Billing event hook system
+# =============================================================================
+
+_billing_hooks: dict[str, list] = {}
+
+
+def on_billing_event(*event_types: str):
+    """Decorator: register a handler for one or more billing event types."""
+    def decorator(func):
+        for event_type in event_types:
+            _billing_hooks.setdefault(event_type, []).append(func)
+        return func
+    return decorator
+
+
+async def _fire_hooks(event_type: str, data: dict) -> None:
+    """Fire all registered hooks for an event type, isolating per-hook failures."""
+    for hook in _billing_hooks.get(event_type, []):
+        try:
+            await hook(event_type, data)
+        except Exception as e:
+            logger.error("Hook %s failed for event %s: %s", hook.__name__, event_type, e)
+
+
 # Blueprint with its own template folder
 bp = Blueprint(
     "billing",