Add Database Models and Repositories for Notification Rules and Channels

## User Story

As a Backend.AI administrator, I want to store notification rules and channel configurations in the database, so that I can dynamically manage notification routing without code deployment.

## Context

This story implements the database layer for Notification Center, providing persistent storage for notification rules, channel configurations (webhook, SMTP, Slack), and notification history.

## Database Schema

### 1. NotificationChannel

Stores channel configurations (where/how to send):

```python
class NotificationChannel(Base):
__tablename__ = "notification_channels"

id = sa.Column(UUID, primary_key=True, default=uuid4)
name = sa.Column(sa.String(255), nullable=False)
description = sa.Column(sa.Text)

# Channel type: webhook, smtp, slack, discord
channel_type = sa.Column(sa.String(50), nullable=False)

# Type-specific configuration (JSONB)
config = sa.Column(JSONB, nullable=False)
# Examples:
# webhook: {url, method, headers, auth, retry}
# smtp: {host, port, from, tls, auth}
# slack: {webhook_url, channel, username, icon}

# Status
enabled = sa.Column(sa.Boolean, default=True)

# Metadata
created_by = sa.Column(UUID, sa.ForeignKey("users.id"))
created_at = sa.Column(sa.DateTime(timezone=True), default=utcnow)
updated_at = sa.Column(sa.DateTime(timezone=True), onupdate=utcnow)
```

### 2. NotificationRule

Defines event → channel routing with template:

```python
class NotificationRule(Base):
__tablename__ = "notification_rules"

id = sa.Column(UUID, primary_key=True, default=uuid4)
name = sa.Column(sa.String(255), nullable=False)
description = sa.Column(sa.Text)

# Event matching
event_type = sa.Column(sa.String(255), nullable=False)
event_filter = sa.Column(JSONB) # Optional: filter conditions

# Routing
channel_id = sa.Column(UUID, sa.ForeignKey("notification_channels.id"))

# Message template (Jinja2)
message_template = sa.Column(sa.Text, nullable=False)

# Priority and control
priority = sa.Column(sa.Integer, default=0)
enabled = sa.Column(sa.Boolean, default=True)

# Metadata
created_by = sa.Column(UUID, sa.ForeignKey("users.id"))
created_at = sa.Column(sa.DateTime(timezone=True), default=utcnow)
updated_at = sa.Column(sa.DateTime(timezone=True), onupdate=utcnow)

# Relationship
channel = relationship("NotificationChannel")
```

### 3. NotificationLog (Optional, for history)

```python
class NotificationLog(Base):
__tablename__ = "notification_logs"

id = sa.Column(UUID, primary_key=True, default=uuid4)
event_id = sa.Column(sa.String(255), nullable=False, index=True)
event_type = sa.Column(sa.String(255), nullable=False)

rule_id = sa.Column(UUID, sa.ForeignKey("notification_rules.id"))
channel_id = sa.Column(UUID, sa.ForeignKey("notification_channels.id"))

# Delivery status
status = sa.Column(sa.String(50)) # pending, sent, failed
error_message = sa.Column(sa.Text)
retry_count = sa.Column(sa.Integer, default=0)

# Timestamps
created_at = sa.Column(sa.DateTime(timezone=True), default=utcnow)
sent_at = sa.Column(sa.DateTime(timezone=True))
```

## Migration Script

Create Alembic migration for all tables with proper indexes.

## Repository Layer

Implement repository pattern for data access:

```python
class NotificationChannelRepository:
async def create(self, channel: NotificationChannel) -> NotificationChannel
async def get_by_id(self, channel_id: UUID) -> NotificationChannel | None
async def list_enabled(self) -> list[NotificationChannel]
async def update(self, channel_id: UUID, updates: dict) -> NotificationChannel
async def delete(self, channel_id: UUID) -> None

class NotificationRuleRepository:
async def create(self, rule: NotificationRule) -> NotificationRule
async def get_by_id(self, rule_id: UUID) -> NotificationRule | None
async def find_by_event_type(self, event_type: str) -> list[NotificationRule]
async def list_enabled(self) -> list[NotificationRule]
async def update(self, rule_id: UUID, updates: dict) -> NotificationRule
async def delete(self, rule_id: UUID) -> None
```

## Acceptance Criteria

- [ ] Database models defined with proper types
- [ ] Alembic migration created and tested
- [ ] JSONB validation for channel config
- [ ] Repository layer with CRUD operations
- [ ] Indexes on event_type and enabled columns
- [ ] Foreign key constraints properly set
- [ ] Unit tests for repositories
- [ ] Sample data for testing

## Related Issues

Epic: BA-302

JIRA Issue: BA-2862