fix(notifier): detach ScheduleImmediate async work from request context

[notque]

Summary

• Notifier.ScheduleImmediate schedules a gocron one-shot job that runs after the HTTP handler returns. It captured params.HTTPRequest.Context(), which net/http cancels at handler return — so the async DB lookup and Campfire HTTP call both failed with context canceled. Every immediate endpoint notification was silently dropped in production.
• Use gocron's native func(ctx context.Context) task signature so the task receives a job-scoped context from the scheduler. That context is independent of the caller's HTTP request lifetime AND is cancelled when the scheduler shuts down, so in-flight tasks short-circuit cleanly on shutdown.
• Wrap the task ctx with a 30s context.WithTimeout so a stuck DB query or Campfire HTTP call cannot run unbounded and stall scheduler shutdown.
• Add a regression test that cancels the parent context before the gocron task fires. Without the fix, the test fails with async notification job did not complete after parent context was cancelled.
• Rewrite the existing ScheduleImmediate test to use a buffered channel instead of assert.Eventually over a shared variable, which was tripping -race.

Why this matters

Found during review of #772. Affects every endpoint approval path that calls ScheduleImmediate — the user-visible symptom is missing Campfire/email notifications with no error returned to the API caller (the handler had already responded 200).

Review-driven evolution

Initial fix used context.WithoutCancel(ctx) to detach cancellation while preserving request-scoped values. Per @notandy's review the implementation was simplified to gocron's job context, which already provides caller-detachment AND adds graceful shutdown cancellation that WithoutCancel did not. Verified via grep that no caller in internal/controller/ or internal/middlewares/ propagates request-scoped ctx values into the notifier task body, so the change is value-lossless.

Test plan

• go test -race -count=3 ./internal/notifier/... — passes
• Regression test (TestNotifier_ScheduleImmediate_ParentContextCancelled) fails on main, passes here — RED-GREEN verified
• make check linters: shellcheck, golangci-lint (0 issues), go-licence-detector, addlicense, reuse — all green

[Copilot]

Pull request overview

This PR updates notifier immediate scheduling so asynchronous notification jobs are detached from HTTP request cancellation while preserving request-scoped context values, and adds regression coverage around parent context cancellation.

Changes:

• Uses context.WithoutCancel for ScheduleImmediate DB lookup and notification send.
• Reworks the existing immediate notification test to use a channel.
• Adds a cancellation regression test for async notification jobs.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
internal/notifier/notifier.go	Detaches the async notification context from caller cancellation.
internal/notifier/notifier_test.go	Updates immediate notification testing and adds parent-cancellation coverage.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (1)

internal/notifier/notifier_test.go:185

• The test HTTP handler ignores JSON decode errors (_ = Decode(...)), which can hide failures and make this regression test non-diagnostic. Prefer require.NoError/assert.NoError on Decode so a malformed request body fails the test immediately.

	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		var req CampfireRequest
		_ = json.NewDecoder(r.Body).Decode(&req)
		w.WriteHeader(http.StatusOK)
		received <- req
	}))

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/sapcc/archer/internal/notifier	0.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/sapcc/archer/internal/notifier/notifier.go	0.00% (ø)	0	0	0

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/sapcc/archer/internal/notifier/notifier_test.go