feat: TR-069 Exchange Viewer - structured MongoDB storage with SSE streaming and sequence diagram UI

- MongoDB storage: cwmp_exchanges collection with indexes (deviceId+timestamp, deviceId+sequenceNumber unique, TTL 24h)
- ExchangeStoreService: CWMP method extraction, per-device queue for ordering, retry logic, message cap (5000/device)
- Exchange API: GET/DELETE/SSE stream endpoints with Last-Event-ID reconnection
- CwmpLoggerMiddleware: dual-write (MongoDB + legacy files) with fire-and-forget
- Frontend: SequenceViewer (CPE↔ACS lifelines, arrows, auto-scroll, virtual scroll) + InspectorPanel (formatted XML, copy, syntax highlighting)
- Tr069Trace page: integrated new viewer with col-6/col-6 layout
- DeviceDetail page: collapsible exchange section when device is traced
- 14 property-based tests, 40+ unit tests, 5 integration tests
- Health check includes index verification
- Config: feature flags, SSE timeouts, retry policy

database/mongo_migrations/011_create_cwmp_exchanges_collection.ts

@@ -0,0 +1,33 @@
+import { MongoMigration } from '../mongo_migrator.js'
+
+export default class CreateCwmpExchangesCollection extends MongoMigration {
+  async up(): Promise<void> {
+    const exchanges = this.db.collection('cwmp_exchanges')
+
+    // Compound index for efficient retrieval by device, ordered by time
+    await exchanges.createIndex(
+      { deviceId: 1, timestamp: 1 },
+      { name: 'idx_deviceId_timestamp' }
+    )
+
+    // Unique index for atomic sequence numbering and Last-Event-ID queries
+    await exchanges.createIndex(
+      { deviceId: 1, sequenceNumber: 1 },
+      { unique: true, name: 'idx_deviceId_sequenceNumber' }
+    )
+
+    // TTL index to auto-remove exchanges older than 24h
+    await exchanges.createIndex(
+      { createdAt: 1 },
+      { expireAfterSeconds: 86400, name: 'idx_createdAt_ttl' }
+    )
+
+    // Create counters collection for atomic sequence number generation
+    this.db.collection('cwmp_exchange_counters')
+  }
+
+  async down(): Promise<void> {
+    await this.db.dropCollection('cwmp_exchanges').catch(() => {})
+    await this.db.dropCollection('cwmp_exchange_counters').catch(() => {})
+  }
+}