refactor: consolidate extension system to general-only with manifest-driven architecture

- Remove all sector-specific extensions (construction, ecommerce, export,
  hotel, restaurant, tech) — only general-purpose extensions remain
- Move NE-bilaga and SRU export from extensions to core reports (lib/reports/)
- Move moms-box-mapping from extensions/export/shared to lib/vat/
- Replace per-extension API routes with catch-all dispatcher
  (app/api/extensions/ext/[...path]/route.ts)
- Add manifest.json for each extension with metadata, env vars, and deps
- Add api-routes.ts pattern for extension-defined API endpoints
- Add code generation scripts (generate-extension-registry, create-extension)
- Add extensions.config.json for opt-in extension loading
- Add extensions.schema.json for config validation
- Add email service interface with noop default (lib/email/service.ts)
- Add CI workflow (core-build.yml) to verify core builds with zero extensions
- Add migration 045: expand account_type CHECK for untaxed_reserves
- Update CLAUDE.md with comprehensive extension system documentation
- Update all report engines and bookkeeping services for new imports
- Clean up extensions.schema.json to only list existing extensions

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

extensions.schema.json

@@ -0,0 +1,34 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "erp-base Extension Configuration",
+  "description": "Configure which extensions are enabled. Run `npm run setup:extensions` after modifying.",
+  "type": "object",
+  "required": [
+    "extensions"
+  ],
+  "properties": {
+    "$schema": {
+      "type": "string"
+    },
+    "extensions": {
+      "type": "array",
+      "uniqueItems": true,
+      "items": {
+        "type": "string",
+        "enum": [
+          "receipt-ocr",
+          "ai-categorization",
+          "ai-chat",
+          "push-notifications",
+          "invoice-inbox",
+          "calendar",
+          "enable-banking",
+          "email",
+          "user-description-match"
+        ]
+      },
+      "description": "Extension IDs to enable. Each ID must match a manifest.json in the extensions/ directory."
+    }
+  },
+  "additionalProperties": false
+}