feat(server): Add ListObservers RPC

[devsjc]

• Have you followed the Open Climate Fix Contribution Guidelines?
• Have you referenced the Issue this PR addresses?
• 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?.
• Have you successfully run make lint with your changes locally?
• Have you successfully run make gen with your changes locally?
• Have you successfully run make test with your changes locally?

Adds a ListObservers RPC, sures up validation and logging.

[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":"2025-11-07T11:50:59Z","message":"Running migrations"}
goos: linux
goarch: amd64
pkg: github.com/openclimatefix/data-platform/internal/server/postgres
cpu: AMD EPYC 7763 64-Core Processor                
BenchmarkPostgresClient/6144000/GetForecastAsTimeseries-4         	     189	   5897279 ns/op
BenchmarkPostgresClient/6144000/GetForecastAtTimestamp-4          	      43	  25795116 ns/op
BenchmarkPostgresClient/6144000/GetObservationsAsTimeseries-4     	     931	   1229443 ns/op
BenchmarkPostgresClient/6144000/CreateForecast-4                  	     334	   3272475 ns/op
PASS
ok  	github.com/openclimatefix/data-platform/internal/server/postgres	110.133s
?   	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-list-observers.txt    │
                                                     │     sec/op     │    sec/op     vs base                 │
PostgresClient/6144000/GetForecastAsTimeseries-4         4.826m ± ∞ ¹   5.897m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetForecastAtTimestamp-4         105.00m ± ∞ ¹   25.80m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/GetObservationsAsTimeseries-4     1.307m ± ∞ ¹   1.229m ± ∞ ¹        ~ (p=1.000 n=1) ²
PostgresClient/6144000/CreateForecast-4                  2.612m ± ∞ ¹   3.272m ± ∞ ¹        ~ (p=1.000 n=1) ²
geomean                                                  6.449m         4.974m        -22.88%
¹ need >= 6 samples for confidence interval at level 0.95
² need >= 4 samples to detect a difference at alpha level 0.05