feat: add comprehensive per-service statistics tracking

Add full support for tracking and analyzing data by service in multi-tenant deployments.

## Features Added

- **Service Statistics Tracking**: Track nouns, verbs, and metadata counts per service
- **Service Activity Monitoring**: Track first/last activity timestamps and operation counts
- **New API Methods**:
  - `listServices()`: List all services with their statistics and status
  - `getServiceStatistics(service)`: Get detailed stats for a specific service
  - Enhanced `getStatistics()` with service filtering and breakdown

- **Service Filtering**: Filter search results and queries by service
- **Storage Enhancements**: BaseStorageAdapter tracks service activity with timestamps
- **Type Definitions**: Added ServiceStatistics interface and extended StatisticsData

## Implementation Details

- Services automatically tracked via defaultService config or per-operation override
- Service status detection (active/inactive/read-only) based on activity
- Memory-efficient tracking at statistics level, not per noun/verb
- Backward compatible - existing data tracked under 'default' service

## Documentation

- Comprehensive guide in docs/guides/per-service-statistics.md
- Examples for multi-tenant apps, health monitoring, and auditing
- API reference and migration guide included

## Testing

- Full test suite in tests/service-statistics.test.ts
- Coverage of all new methods and filtering capabilities

This enables better observability, debugging, and management of multi-service Brainy deployments, addressing the need to track individual service performance when multiple services share storage.

src/storage/adapters/memoryStorage.ts

@@ -622,6 +622,16 @@ export class MemoryStorage extends BaseStorage {
       metadataCount: {...statistics.metadataCount},
       hnswIndexSize: statistics.hnswIndexSize,
       lastUpdated: statistics.lastUpdated,
+      // Include serviceActivity if present
+      ...(statistics.serviceActivity && {
+        serviceActivity: Object.fromEntries(
+          Object.entries(statistics.serviceActivity).map(([k, v]) => [k, {...v}])
+        )
+      }),
+      // Include services if present
+      ...(statistics.services && {
+        services: statistics.services.map(s => ({...s}))
+      }),
       // Include distributedConfig if present
       ...(statistics.distributedConfig && { 
         distributedConfig: JSON.parse(JSON.stringify(statistics.distributedConfig)) 
@@ -648,6 +658,16 @@ export class MemoryStorage extends BaseStorage {
       metadataCount: {...this.statistics.metadataCount},
       hnswIndexSize: this.statistics.hnswIndexSize,
       lastUpdated: this.statistics.lastUpdated,
+      // Include serviceActivity if present
+      ...(this.statistics.serviceActivity && {
+        serviceActivity: Object.fromEntries(
+          Object.entries(this.statistics.serviceActivity).map(([k, v]) => [k, {...v}])
+        )
+      }),
+      // Include services if present
+      ...(this.statistics.services && {
+        services: this.statistics.services.map(s => ({...s}))
+      }),
       // Include distributedConfig if present
       ...(this.statistics.distributedConfig && { 
         distributedConfig: JSON.parse(JSON.stringify(this.statistics.distributedConfig))