fix(sql): Optimize core forecast fetching

[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

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Changes in this Pull Request

Further optimizations to the forecast SQL.

[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":"2026-03-24T19:25:39Z","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         	      67	  16420455 ns/op
BenchmarkPostgresClient/small/GetForecastAtTimestamp-4          	      85	  13942000 ns/op
BenchmarkPostgresClient/small/GetObservationsAsTimeseries-4     	     894	   1224670 ns/op
BenchmarkPostgresClient/small/CreateForecast-4                  	      90	  12956216 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	77.506s
?   	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-more-optimizations.txt  │
                                                   │     sec/op     │    sec/op     vs base                │
PostgresClient/small/GetForecastAsTimeseries-4         15.75m ± ∞ ¹   16.42m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/GetForecastAtTimestamp-4          12.57m ± ∞ ¹   13.94m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/GetObservationsAsTimeseries-4     1.203m ± ∞ ¹   1.225m ± ∞ ¹       ~ (p=1.000 n=1) ²
PostgresClient/small/CreateForecast-4                  12.69m ± ∞ ¹   12.96m ± ∞ ¹       ~ (p=1.000 n=1) ²
geomean                                                7.415m         7.763m        +4.70%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05