Implement Notification Center Core with Redis Stream and Worker Architecture

## User Story

As a Backend.AI system administrator, I want a centralized notification processing system that can handle events asynchronously via Redis Stream and process them through worker pools, so that notification delivery is reliable and scalable.

## Context

This story implements the foundational infrastructure for the Notification Center, establishing the event stream, worker architecture, and core processing logic that other components will build upon.

## Functional Requirements

### 1. Redis Stream Management

Implement event stream for notification events:

```python
# Stream configuration
STREAM_KEY = "notification:events"
STREAM_MAXLEN = 500 # Approximate
CONSUMER_GROUP = "notification-workers"
```

**Operations**:

- Event publishing (XADD)
- Consumer group creation
- Message acknowledgment (XACK)
- Stream monitoring

### 2. NotificationEvent Data Model

Define the standardized event object:

```python
@dataclass
class NotificationEvent:
event_id: str
event_type: str # "bgtask.completed", "quota.warning", etc.
timestamp: datetime
source: str # "manager", "agent", "gateway"

# Content
subject: str
body: str | None
severity: Literal["info", "warning", "error", "critical"]

# Structured data for templates
context: dict[str, Any]
metadata: dict[str, Any]

# Optional UI elements
action_url: str | None
actions: list[dict] | None
```

### 3. Worker Pool Implementation

### 4. Event Publisher API

### 5. Handler Registry

### 6. Configuration

## Acceptance Criteria

- [ ] NotificationEvent dataclass with all fields defined
- [ ] Redis Stream operations (XADD, XREADGROUP, XACK)
- [ ] Consumer group auto-creation on startup
- [ ] Worker pool implementation with configurable size
- [ ] Event publisher API with batch support
- [ ] Handler registry with registration/retrieval
- [ ] Configuration in manager.toml
- [ ] Graceful shutdown handling
- [ ] Error isolation and retry logic
- [ ] Unit tests with >80% coverage

## Related Issues

Epic: BA-302

JIRA Issue: BA-2861