feat(proto): GetObservationsAtTimestamp route

[devsjc]

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.

Changes in this Pull Request

Adds a GetObservationsAtTimestamp route. This will speed up pvlive/all, which currently takes around 11 seconds.

[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":"2026-03-20T15:58:39Z","message":"Completed migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
BenchmarkPostgresClient/small/GetForecastAsTimeseries-4         	      57	  21092857 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	      75	  15726027 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	    1050	   1131173 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	      84	  13871288 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	65.520s
?   	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 9V74 80-Core Processor                
                                                   │ bench-main.txt │
                                                   │     sec/op     │
PostgresClient/small/GetForecastAsTimeseries-4         23.09m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4          16.30m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4     1.225m ± ∞ ¹
PostgresClient/small/CreateForecast-4                  14.10m ± ∞ ¹
geomean                                                8.980m
¹ need >= 6 samples for confidence interval at level 0.95

cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
                                                   │ bench-devsjc-observations-snapshot.txt │
                                                   │                 sec/op                 │
PostgresClient/small/GetForecastAsTimeseries-4                                 21.09m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4                                  15.73m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4                             1.131m ± ∞ ¹
PostgresClient/small/CreateForecast-4                                          13.87m ± ∞ ¹
geomean                                                                        8.494m
¹ need >= 6 samples for confidence interval at level 0.95