Add sensor datapoint access with linear interpolation support

[Copilot]

This PR implements a new GraphQL API for accessing single sensor datapoints before, after, or interpolated at a given datetime value, addressing the need for precise sensor data retrieval at specific timestamps.

New Features

GraphQL API

Added a new sensor_datapoint resolver that supports three access modes:

query GetSensorDatapoint($sensorId: String!, $targetTime: DateTime!, $direction: String!) {
  sensorDatapoint(sensorId: $sensorId, targetTime: $targetTime, direction: $direction) {
    value
    timestamp
    isInterpolated
    sourceReadings {
      timestamp
      value
    }
  }
}

Direction modes:

• "before" - Returns the last reading before the target time
• "after" - Returns the first reading after the target time
• "interpolate" - Returns a linearly interpolated value at the target time

Linear Interpolation

The interpolation feature calculates time-weighted averages between the closest readings:

interpolated_value = before_value + factor * (after_value - before_value)
factor = (target_time - before_time) / (after_time - before_time)

This enables smooth data access for applications like Heat Pump COP calculations where precise values at period boundaries are needed.

Implementation Details

• Backend: New SingleDatapoint GraphQL type with interpolation metadata
• Frontend: Added GET_SENSOR_DATAPOINT query for easy integration
• Tests: Comprehensive test suite covering all modes and edge cases
• Performance: Efficient database queries using proper indexing and limits

Use Cases

This feature is particularly valuable for:

• Heat pump efficiency analysis requiring precise energy readings at period boundaries
• Historical data analysis at specific timestamps
• Data smoothing and gap filling in time series
• Threshold monitoring at exact times

Fixes #5.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

@intui 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.