Fix/detached forecasting sensors

[BelhsanHmida]

Description

Previously, train/predict cycle jobs enqueued a bound TrainPredictPipeline method, which caused the pipeline instance and its ORM-backed sensors/data source to be pickled and sent across the web-worker boundary. In the worker, those objects could be detached from the active DB session and fail with DetachedInstanceError, especially for regressors.

This PR changes cycle jobs to pass only serialized config/parameters plus data_source_id, then reconstructs the pipeline inside the worker. This keeps ORM objects session-local to the worker and avoids carrying session-bound state through RQ.

• Serialize queued train-predict cycle jobs with plain IDs and timing/config payloads.
• Reconstruct target sensor, save sensor, regressors, and data source inside the worker DB session.
• Commit the forecasting DataSource before enqueuing jobs so workers can load it by ID.
• Keep queued cycle jobs limited to serialized pipeline state and per-cycle timings; arbitrary job_kwargs are intentionally not forwarded.
• Add regression coverage proving queued job kwargs do not contain ORM instances.
• Add changelog entry.

Closes #2256

How to test

uv run pytest flexmeasures/data/tests/test_forecasting_pipeline.py flexmeasures/data/tests/test_forecasting_jobs.py flexmeasures/data/tests/test_forecasting_jobs_fresh_db.py flexmeasures/api/v3_0/tests/test_forecasting_api_fresh_db.py flexmeasures/data/tests/test_job_cache.py -q

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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

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Pull request overview

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