feat(server): Add GetLatestObservation RPC

[devsjc]

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Implements GetLatestObservation RPC for fetching the last observed value before a given time for a location. Part of #18

[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	0.004s
{"level":"debug","time":"2025-11-05T12:08:09Z","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         	     219	   5156752 ns/op
BenchmarkPostgresClient/6144000/GetForecastAtTimestamp-4          	      42	 105512882 ns/op
BenchmarkPostgresClient/6144000/GetObservationsAsTimeseries-4     	     919	   1238934 ns/op
BenchmarkPostgresClient/6144000/CreateForecast-4                  	     464	   2496892 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	112.357s
?   	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-latest-observation.txt  │
                                                     │     sec/op     │    sec/op     vs base                │
PostgresClient/6144000/GetForecastAsTimeseries-4         4.826m ± ∞ ¹   5.157m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetForecastAtTimestamp-4          105.0m ± ∞ ¹   105.5m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetObservationsAsTimeseries-4     1.307m ± ∞ ¹   1.239m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/6144000/CreateForecast-4                  2.612m ± ∞ ¹   2.497m ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                                  6.449m         6.405m        -0.68%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05