fix(sql): Faster forecast delete

[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

Updates the DeleteForecast function to better use the index on the PredictedGenerationValues table for speedier deletion.

[peterdudfield]

Good example of here, to use the data-platform api rather than raw 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":"2026-03-05T11:38:41Z","message":"Completed migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkPostgresClient/2400000/GetForecastAsTimeseries-4         	      54	  21141395 ns/op
BenchmarkPostgresClient/2400000/GetForecastAtTimestamp-4          	      51	  21646177 ns/op
BenchmarkPostgresClient/2400000/GetObservationsAsTimeseries-4     	     938	   1198792 ns/op
BenchmarkPostgresClient/2400000/CreateForecast-4                  	      50	  24370398 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	1312.977s
?   	github.com/openclimatefix/data-platform/internal/server/postgres/gen	[no test files]