feat(forecasts): Add metadata column

[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

Adds a metadata column to the forecasts table, and corresponding parameters in the CreateForecast routes, and other routes that return forecast values. Returned forecast value metadata is the combination of the predicted generation values metadata and the forecast metadata.

[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":"2026-02-25T13:06:44Z","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         	      52	  21675322 ns/op
BenchmarkPostgresClient/2400000/GetForecastAtTimestamp-4          	      54	  21546607 ns/op
BenchmarkPostgresClient/2400000/GetObservationsAsTimeseries-4     	     960	   1206239 ns/op
BenchmarkPostgresClient/2400000/CreateForecast-4                  	      50	  22686961 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	1300.522s
?   	github.com/openclimatefix/data-platform/internal/server/postgres/gen	[no test files]