feat(api): Batch StreamForecastData responses

[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?

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Changes in this Pull Request

Modifies the StreamForecastData route to return its values in batches. This should be more stable and reduce CPU usage a bit. It does introduce a breaking change to the API, but since the Stream route is not on any hot path, I don't mind doing this. This is still in pre-1.0.0 after all.

The change is reflected in the example.py file, and boils down to data from the stream being under a values field, when previously it was returned top-level.

[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":"2026-05-06T15:38:11Z","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         	      61	  18344513 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	     307	   3769575 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	    1090	   1060037 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	     100	  11638655 ns/op
BenchmarkPostgresClient/small/StreamForecastData-4              	      14	  83146088 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	66.469s
?   	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 │
                                                   │     sec/op     │
PostgresClient/small/GetForecastAsTimeseries-4         16.32m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4          4.162m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4     1.265m ± ∞ ¹
PostgresClient/small/CreateForecast-4                  13.20m ± ∞ ¹
PostgresClient/small/StreamForecastData-4              96.92m ± ∞ ¹
geomean                                                10.19m
¹ need >= 6 samples for confidence interval at level 0.95

cpu: Intel(R) Xeon(R) Platinum 8370C CPU @ 2.80GHz
                                                   │ bench-devsjc-batched-streaming.txt │
                                                   │               sec/op               │
PostgresClient/small/GetForecastAsTimeseries-4                             18.34m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4                              3.770m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4                         1.060m ± ∞ ¹
PostgresClient/small/CreateForecast-4                                      11.64m ± ∞ ¹
PostgresClient/small/StreamForecastData-4                                  83.15m ± ∞ ¹
geomean                                                                    9.336m
¹ need >= 6 samples for confidence interval at level 0.95