test(handler): add unit tests for groups.go

[damnnmanyaa]

Addresses #1581, scoped to groups.go

What's covered

Unit tests for all 11 exported handler functions in backend/pkg/handler/groups.go, using httptest + echo.Context against a real seeded Postgres database via api.NewForTest matching the existing pattern already established in this codebase (backend/pkg/api/metrics_test.go).

Each handler is covered for:

• A successful request (correct status code + response shape)
• A not-found or app-not-found case where applicable
• A malformed/validation-failure case (invalid ID, invalid duration, malformed JSON body)

Note for reviewers: nonexistent group IDs on aggregate endpoints

The group aggregate endpoints currently return 200 with empty data when given a syntactically valid but nonexistent group ID :-

• GET .../version_timeline
• GET .../status_timeline
• GET .../version_breakdown

These handlers query aggregate data tables directly rather than resolving the group first (unlike GetGroup, UpdateGroup, and DeleteGroup, which query the groups table via dbreads.Queries.GetGroup and 404 on sql.ErrNoRows). As a result, a nonexistent group with no matching instance or status-history rows is currently indistinguishable from an
existing group that simply has no data yet.

This is out of scope for this test-coverage PR. The tests reflect actual current behavior rather than assuming what it should be. Happy to open a follow-up issue if maintainers think this is worth addressing, or to leave it as intentional empty-aggregate semantics if that's by design.

Next steps

Plan is to extend this same pattern to the other handler files (channels.go, packages.go, etc.) in follow-up PRs once this one is reviewed.

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