Per-tag diff coverage thresholds

[laura-mlg]

Summary

Allow teams to configure different diff coverage thresholds per coverage tag, rather than applying one workspace-wide threshold to all tags.

Problem

Today, the workspace Review Config applies a single diff coverage threshold to all tags equally. Each tag gets its own pass/fail gate on a PR (e.g. qlty coverage diff / backend, qlty coverage diff / frontend), but they all use the same threshold.

Teams with genuinely different coverage requirements per component — e.g. a backend that requires 80% and a frontend that requires 50% — have no way to express this today.

Proposed Solution

Allow per-tag threshold overrides in the Review Config, so teams can specify different diff coverage thresholds per tag. The existing workspace-level threshold would serve as the default for any tag without an explicit override.