feat: add Valkey integration for task queue (closes #98)

- Add ioredis package dependency for Redis-compatible operations
- Create ValkeyModule as global NestJS module
- Implement ValkeyService with task queue operations:
  - enqueue(task): Add tasks to FIFO queue
  - dequeue(): Get next task and update to PROCESSING status
  - getStatus(taskId): Retrieve task metadata and status
  - updateStatus(taskId, status): Update task state (COMPLETED/FAILED)
  - getQueueLength(): Monitor queue depth
  - clearQueue(): Queue management utility
  - healthCheck(): Verify Valkey connectivity
- Add TaskDto, EnqueueTaskDto, UpdateTaskStatusDto interfaces
- Implement TaskStatus enum (PENDING/PROCESSING/COMPLETED/FAILED)
- Add comprehensive test suite with in-memory Redis mock (20 tests)
- Integrate ValkeyModule into app.module.ts
- Valkey Docker Compose service already configured in docker-compose.yml
- VALKEY_URL environment variable already in .env.example
- Add detailed README with usage examples and API documentation

Technical Details:
- Uses FIFO queue (RPUSH/LPOP for strict ordering)
- Task metadata stored with 24-hour TTL
- Lifecycle hooks for connection management (onModuleInit/onModuleDestroy)
- Automatic retry with exponential backoff on connection errors
- Global module - no explicit imports needed

Tests verify:
- Connection initialization and health checks
- FIFO enqueue/dequeue behavior
- Status lifecycle transitions
- Concurrent task handling
- Queue management operations
- Complete task processing workflows

apps/api/src/valkey/valkey.module.ts

@@ -0,0 +1,16 @@
+import { Module, Global } from '@nestjs/common';
+import { ValkeyService } from './valkey.service';
+
+/**
+ * ValkeyModule - Redis-compatible task queue module
+ * 
+ * This module provides task queue functionality using Valkey (Redis-compatible).
+ * It is marked as @Global to allow injection across the application without
+ * explicit imports.
+ */
+@Global()
+@Module({
+  providers: [ValkeyService],
+  exports: [ValkeyService],
+})
+export class ValkeyModule {}