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/dto/task.dto.ts

@@ -0,0 +1,47 @@
+/**
+ * Task status enum
+ */
+export enum TaskStatus {
+  PENDING = 'pending',
+  PROCESSING = 'processing',
+  COMPLETED = 'completed',
+  FAILED = 'failed',
+}
+
+/**
+ * Task metadata interface
+ */
+export interface TaskMetadata {
+  [key: string]: unknown;
+}
+
+/**
+ * Task DTO for queue operations
+ */
+export interface TaskDto {
+  id: string;
+  type: string;
+  data: TaskMetadata;
+  status: TaskStatus;
+  error?: string;
+  createdAt?: Date;
+  updatedAt?: Date;
+  completedAt?: Date;
+}
+
+/**
+ * Enqueue task request DTO
+ */
+export interface EnqueueTaskDto {
+  type: string;
+  data: TaskMetadata;
+}
+
+/**
+ * Update task status DTO
+ */
+export interface UpdateTaskStatusDto {
+  status: TaskStatus;
+  error?: string;
+  result?: TaskMetadata;
+}