fix: run all server goroutines under one errgroup

[dhyaniarun1993]

What

Unifies goroutine lifecycle management across the api, indexer, and relayer servers under a single errgroup each, and standardizes the background-worker signatures that feed it.

[gemini-code-assist]

Code Review

This pull request refactors the API, indexer, and relayer servers to run all long-lived background workers and HTTP servers under a single shared errgroup.Group. This ensures coordinated shutdown and prevents race conditions with deferred resource cleanups. Accordingly, background tasks (such as workers, miners, and caches) now return an error instead of running as detached goroutines. The reviewer identified a critical issue in pkg/app/api/server.go where a failure in initServices could cause a goroutine leak because the function returns immediately without waiting for already-spawned background tasks to drain. A fix was suggested to cancel the context and wait for the group to finish on error.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 27.58621% with 21 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@9a5ccbd). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pkg/app/api/server.go	0.00%	12 Missing ⚠️
pkg/app/indexer/server.go	0.00%	4 Missing ⚠️
pkg/app/relayer/server.go	0.00%	3 Missing ⚠️
pkg/user/service/topology_cache.go	0.00%	2 Missing ⚠️

❌ Your patch status has failed because the patch coverage (27.58%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #303   +/-   ##
=======================================
  Coverage        ?   30.64%           
=======================================
  Files           ?      150           
  Lines           ?    11275           
  Branches        ?        0           
=======================================
  Hits            ?     3455           
  Misses          ?     7547           
  Partials        ?      273           

Flag	Coverage Δ
unittests	30.64% <27.58%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pkg/custodial/accept_worker.go	95.00% <100.00%> (ø)
pkg/ethrpc/miner/miner.go	97.84% <100.00%> (ø)
pkg/ethrpc/submitter/submitter.go	77.70% <100.00%> (ø)
pkg/transfer/cache.go	94.87% <100.00%> (ø)
pkg/user/service/topology_cache.go	0.00% <0.00%> (ø)
pkg/app/relayer/server.go	0.00% <0.00%> (ø)
pkg/app/indexer/server.go	0.00% <0.00%> (ø)
pkg/app/api/server.go	0.00% <0.00%> (ø)

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