Feat/add fetch forecasts endpoint

[BelhsanHmida]

Description

• Added new endpoint: GET /api/v3_0/sensors/<sensor_id>/forecasts/<job_id>
• Endpoint checks the RQ job status for the given job UUID.
• If the job is still queued or running → returns job status only.
• If the job failed → returns an error with failure info.
• If the job completed successfully → fetches forecasts from the DB and returns them.
• Ensures the sensor in the URL matches the sensor associated with the forecast job.
• Added validation and permission checks for accessing sensor forecasts.
• Added changelog item in documentation/changelog.rst.

Look & Feel

Example response: Job still running

{
  "status": "RUNNING",
  "message": "Forecasting job is still in progress."
}

Example response: Job completed

{
  "status": "DONE",
  "sensor": 2092,
  "forecasts": [
    { "timestamp": "2025-10-15T00:00:00+01:00", "value": 12.5 },
    { "timestamp": "2025-10-15T01:00:00+01:00", "value": 12.8 }
  ]

Example response: Job failed

{
  "status": "FAILED",
  "message": "Forecasting job encountered an error."
}

How to test

• Trigger a forecast first via:
  POST /api/v3_0/sensors/<id>/forecasts/trigger
• Copy the returned job UUID.
• Call:
  GET /api/v3_0/sensors/<id>/forecasts/<uuid>
• Verify all cases:
  • Queued job
  • Running job
  • Finished job (should return forecasts)
  • Failed job (simulate failure)
  • Wrong sensor ID → returns validation error

Related Items

closes #1814

...

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Sign-off

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on code under GPL or other license that is incompatible with FlexMeasures

[read-the-docs-community]

Documentation build overview

📚 flexmeasures | 🛠️ Build #30560313 | 📁 Comparing 8d55ec0 against latest (32939a1)

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File	Status
genindex.html	📝 modified
_autosummary/flexmeasures.api.v3_0.sensors.html	📝 modified
_autosummary/flexmeasures.data.services.scheduling.html	📝 modified
api/v3_0.html	📝 modified