feat(backend): Simplify location ownership

[devsjc]

Adds a much simpler model for handling location ownership. Removes the iam schema entirely and replaces it with an entities table in the location schema that stores the owning entities.

Contribution Checklist

• Have you followed the Open Climate Fix Contribution Guidelines?
• Have you referenced the Issue this PR addresses, where applicable?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added a summary of the changes?
• Have you written new tests for your changes, where applicable?
• Have you successfully run make lint with your changes locally?
• Have you successfully run make test with your changes locally?

Warning

PRs may be closed if all the above boxes are not checked.

[github-actions]

Benchmark Results

Benchmark results

?   	github.com/openclimatefix/data-platform/cmd	[no test files]
?   	github.com/openclimatefix/data-platform/internal/gen/ocf/dp	[no test files]
?   	github.com/openclimatefix/data-platform/internal/interceptors	[no test files]
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/dummy	0.005s
{"level":"debug","time":"2026-06-11T15:12:59Z","message":"Completed migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkPostgresClient/small/GetForecastAsTimeseries-4         	      61	  18545514 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	     264	   4367151 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	     894	   1298880 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	      87	  13186921 ns/op
BenchmarkPostgresClient/small/StreamForecastData-4              	      12	  90244235 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	73.495s
?   	github.com/openclimatefix/data-platform/internal/server/postgres/gen	[no test files]

Benchmark vs base branch

goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
                                                   │ bench-main.txt │     bench-devsjc-org-groups.txt      │
                                                   │     sec/op     │    sec/op     vs base                │
PostgresClient/small/GetForecastAsTimeseries-4         15.76m ± ∞ ¹   18.55m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/GetForecastAtTimestamp-4          4.155m ± ∞ ¹   4.367m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/GetObservationsAsTimeseries-4     1.249m ± ∞ ¹   1.299m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/CreateForecast-4                  12.73m ± ∞ ¹   13.19m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/StreamForecastData-4              83.44m ± ∞ ¹   90.24m ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                                9.723m         10.46m        +7.58%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05