feat: add unified observability endpoint with SLS/DB routing

[githuuuuub]

📝 Description

• Add ObservabilityConfig to switch log source via OBSERVABILITY_LOG_SOURCE env var (default: SLS)
• Add ObservabilityController as unified entry at /observability/statistics
• Route requests to SlsController or DBCollectorController based on config
• Update frontend slsApi.ts to call /observability/statistics
• Log data source details on startup (SLS config or DB URL + table)

🔗 Related Issues

✅ Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Build/CI configuration change
• Other (please describe):

🧪 Testing

• Unit tests added/updated
• Integration tests added/updated
• Manual testing completed
• All tests pass locally

📋 Checklist

• Code has been formatted (mvn spotless:apply for backend, npm run lint:fix for frontend)
• Code is self-reviewed
• Comments added for complex code
• Documentation updated (if applicable)
• No breaking changes (or migration guide provided)
• All CI checks pass

📊 Test Coverage

📚 Additional Notes