Replace findthatpostcode with our own self-hosted postcodes.io

[mbarton]

We have noticed variable performance on findthatpostcode.uk, sometimes up to 10 seconds for a single request. This is fine because we are of course not paying for it! However in practice that's likely to be too slow for our users.

We have discussed creating a self-hosted instance of postcodes.io. We moved away from that provider as it seemed less up to date than findthatpostcode but since then they have made multiple releases and should now be up to date as of August 2024.

It's now up and running at https://api.rcpch.ac.uk/postcodes (API key required).

We will migrate in two steps

1. Create new settings that when set use our postcodes.io
2. Remove the old findthatpostcode code path

That way we're rollback safe and won't immediately break everyones local dev environments.

IMPORTANT - we need to discuss how to handle retired postcodes (3 examples in cohort 6).

• postcodes.io does not return data for them
• findthatpostcode does
• Our census platform does not

[mbarton]

Note the docs need updating when this change is made: https://github.com/rcpch/rcpch-audit-engine/pull/1105/files/3fb2de2aa0a0f48b118dc4c4bf932a2686b15b1d#r1867521080

[mbarton]

I've seen a couple of cases over the past 24 hours where correct postcodes are rejected because findthatpostcode returns a 429 Too Many Requests

[mbarton]

Bumping the priority of this as I've seen another 429 this morning. It would also "fix" #1173 for future patients by disallowing O as the sector when it should be 0.

Looking at the error message the free allocation is per day, so we might be stuck all day now?

[mbarton]

postcodes.io does have a terminated postcode search that returns geo coordinates so for retired postcodes we could get the IMD that way