fix(server): Retry failed scheduled snapshots - #5544

Summary

• determine retry eligibility from the cached scheduling policy instead of the cleared next-snapshot timestamp
• restore the five-minute retry after a failed scheduled snapshot, including while the repository is unavailable
• keep manual-only policies unscheduled and add regression coverage for both cases

Fixes #5521

Testing

• go test ./internal/server -run '^TestSourceManagerBackoffAfterFailed(Scheduled|Manual)Snapshot$' -count=10
• go test ./internal/server -count=1
• go test -race ./internal/server -count=1
• go vet ./internal/server
• make check-locks
• tools/.tools/golangci-lint-2.6.1/golangci-lint --timeout 1200s run ./internal/server/...
• git diff --check