Implement Data Retention Policy

[zven]

Discussed in #726

^{Originally posted by scheidtdav February 3, 2026}
OSEM needs a data retention policy in order to be sustainable in operation.
Thankfully TimescaleDB comes with a feature for this.

What are reasonable data retention policies and where do we communicate them?
Should we add continuous aggregates?

Here is a first draft as subject for discussion:

• Measurement data
  • Full resolution for 12 months
  • Hourly averages for younger than 18 months
  • Daily averages for data younger than 24
  • Drop data past 24 months

Any opinions?

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Long story short: We decided to implement the data retention policy as discussed above.