perf(postgres): Optimize table

[devsjc]

Modifies the schema of the predicted generation values table for optimal storage. Reduces table size by about 40%.

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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[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":"2026-06-16T12:18:39Z","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         	      43	  26461048 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	     147	   7739708 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	     852	   1359624 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	      92	  12620555 ns/op
BenchmarkPostgresClient/small/StreamForecastData-4              	      13	  87458785 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	71.435s
?   	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-devsjc-optimize-storage.txt │
                                                   │              sec/op               │
PostgresClient/small/GetForecastAsTimeseries-4                            26.46m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4                             7.740m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4                        1.360m ± ∞ ¹
PostgresClient/small/CreateForecast-4                                     12.62m ± ∞ ¹
PostgresClient/small/StreamForecastData-4                                 87.46m ± ∞ ¹
geomean                                                                   12.52m
¹ need >= 6 samples for confidence interval at level 0.95

cpu: AMD EPYC 9V74 80-Core Processor                
                                                   │ bench-main.txt │
                                                   │     sec/op     │
PostgresClient/small/GetForecastAsTimeseries-4         18.67m ± ∞ ¹
PostgresClient/small/GetForecastAtTimestamp-4          4.202m ± ∞ ¹
PostgresClient/small/GetObservationsAsTimeseries-4     1.205m ± ∞ ¹
PostgresClient/small/CreateForecast-4                  11.74m ± ∞ ¹
PostgresClient/small/StreamForecastData-4              102.4m ± ∞ ¹
geomean                                                10.26m
¹ need >= 6 samples for confidence interval at level 0.95