chore(ci): raise backend coverage job timeout

[Wikid82]

Backend coverage job runs take 12-14 min (internal/services ~9.5m, handlers ~7m under -race), so the 15m job cap leaves almost no headroom (run 37414531459 finished all steps then hit the cap at 15m02s) and Go's default 10m per-package timeout is close to being hit.

• codecov-upload.yml: backend job cap 15 -> 30m, frontend 15 -> 25m (it runs ~12m); agent unchanged (~25s).
• go-test-coverage.sh: pass -timeout to both gotestsum and go test branches, configurable via CHARON_TEST_TIMEOUT (default 30m); documented in the skill file.

Claude Code was used to help produce this change.

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