feat: make extension system packageable via enriched ExtensionContext

Enrich ExtensionContext with supabase, emit(), settings, storage, log,
and services so extensions can receive everything through dependency
injection instead of importing core modules directly.

- Add context factory and inject context into event handlers via registry
- Move supplier invoice journal entry creation to core event handler
- Add services.ingestTransactions to ExtensionContext for enable-banking
- Create catch-all API route for extension-declared apiRoutes
- Migrate 5 extensions to accept context with dynamic import fallbacks

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

lib/init.ts

@@ -1,10 +1,21 @@
 import { loadExtensions } from '@/lib/extensions/loader'
+import { setContextFactory } from '@/lib/extensions/registry'
+import { createExtensionContext } from '@/lib/extensions/context-factory'
+import { registerSupplierInvoiceHandler } from '@/lib/bookkeeping/handlers/supplier-invoice-handler'
+
+let initialized = false
 
 /**
- * Ensure the system is initialized (extensions loaded).
+ * Ensure the system is initialized (extensions loaded, context factory wired,
+ * core event handlers registered).
  * Called from API routes that emit events.
  * Idempotent — safe to call multiple times.
  */
 export function ensureInitialized(): void {
+  if (initialized) return
+  initialized = true
+
+  setContextFactory(createExtensionContext)
+  registerSupplierInvoiceHandler()
   loadExtensions()
 }