Cache: avoid saving object references

[braddf]

Pull Request

Description

Hopefully this is a demonstrable fix for the API cache memory leak. Looks like our custom cache middleware saves references to full Pydantic/SQLAlchemy objects etc. directly from the route handlers that are not cleaned up, and can be multiple times the size of the intended cached responses. These are orphaned when the cache is "cleared" and build up over time. This PR solves this by serialising the objects before caching them, severing the refs and ensuring they aren't swept under the rug in the first place.

Fixes #

How Has This Been Tested?

• Tested on local API to dev DB and many concurrent API requests across large-response endpoints with Postman.

Postman Results

• Fresh from API restart

• Before fix, after making a string of API calls e.g. /forecast/all or /national/forecast with diff query params

• Same, further calls made

• Next API call after cache should have expired, and even with an explicit garbage collection – memory allocations persist

• After serialisation fix, API calls made with diff query params over short period; attributed to FastAPI encoder

• Same, further varying API calls within 2 min period

• After cache expires, memory freed and values much lower

If your changes affect data processing, have you plotted any changes? i.e. have you done a quick sanity check?

• Yes

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked my code and corrected any misspellings

[peterdudfield]

nice, this will be useful

[peterdudfield]

nice comments here btw