Persist recorded sensor values with collision handling

[BelhsanHmida]

Add the backend service and endpoint needed to validate and persist values recorded on a sensor graph, including values produced by recurrence expansion.

Acceptance criteria

• Operate only on the sensor identified by the sensor page or endpoint.
• Require the appropriate permission to create sensor data.
• Validate event starts, values, resolution, timezone, and belief time.
• Use the selected belief time, defaulting to the current time when none is supplied.
• Allow values for the same event start when their belief times differ.
• When an exact event-start and belief-time collision exists, preserve the existing belief.
• Return a summary of values created, skipped because of collisions, and rejected.
• Persist the batch atomically so partial failures do not leave an incomplete recording.
• Reuse the recurrence-expansion function rather than implementing DST behavior in the route.

Tests

• Successful single-value and batch recording.
• Permission failure.
• Same event start with a different belief time.
• Exact collision where the existing belief wins.
• Validation failure rolls back the full batch.

Out of scope

• Graph recording controls.
• Recurrence stored as schema rather than concrete beliefs.