feat(server): GetLatestObservations multiple locs

[devsjc]

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

Modifies the GetLAtestObservations route such that multiple locations can be passed to it at once.

[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":"2025-11-17T11:06:23Z","message":"Running migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkPostgresClient/6144000/GetForecastAsTimeseries-4         	     164	   6953444 ns/op
BenchmarkPostgresClient/6144000/GetForecastAtTimestamp-4          	      43	  24952719 ns/op
BenchmarkPostgresClient/6144000/GetObservationsAsTimeseries-4     	     925	   1243349 ns/op
BenchmarkPostgresClient/6144000/CreateForecast-4                  	     262	   4186154 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	185.261s
?   	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-observations-plural.txt  │
                                                     │     sec/op     │    sec/op     vs base                 │
PostgresClient/6144000/GetForecastAsTimeseries-4         4.826m ± ∞ ¹   6.953m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetForecastAtTimestamp-4         105.00m ± ∞ ¹   24.95m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetObservationsAsTimeseries-4     1.307m ± ∞ ¹   1.243m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/CreateForecast-4                  2.612m ± ∞ ¹   4.186m ± ∞ ¹        ~ (p=1.000 n=1) ²
geomean                                                  6.449m         5.482m        -15.00%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05