Add plugin platform with modular backend and frontend registry.

Split graph/import/meta into Django apps, add API v1 with OpenAPI and pytest, and introduce plugin registry with the tags reference plugin on both FE and BE.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/application/services/graphDataService.test.js

@@ -0,0 +1,26 @@
+import { describe, it, expect, vi } from 'vitest'
+
+vi.mock('../../infrastructure/config/dataMode', () => ({
+  isLocalMode: () => true,
+}))
+
+vi.mock('../../core/pluginRegistry', () => ({
+  applyGraphExtensions: (_kind, payload) => payload,
+}))
+
+vi.mock('../usecases/graph', () => ({
+  getGraphBundle: vi.fn(async () => ({
+    nodes: [{ id: '1', label: 'A' }],
+    edges: [],
+    relationTypes: [],
+  })),
+}))
+
+describe('graphDataService', () => {
+  it('fetchGraphBundle returns nodes from local use case', async () => {
+    const { fetchGraphBundle } = await import('./graphDataService.js')
+    const bundle = await fetchGraphBundle()
+    expect(bundle.nodes).toHaveLength(1)
+    expect(bundle.nodes[0].label).toBe('A')
+  })
+})