ci: scoped 100% coverage gates + policy doc

docs/coverage-policy.md

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+# Coverage policy (CI gate)
+
+## Why scoped coverage gates?
+
+Today, overall repository coverage is low (especially for API routes and Next pages), but we still want CI to **enforce quality deterministically**.
+
+So we start with a strict gate (100% statements + branches) on a **small, explicitly scoped** set of modules that are:
+
+- unit-testable without external services
+- stable and high-signal for regressions
+
+We then expand the gated scope as we add tests.
+
+## Backend scope (100% required)
+
+Enforced in `Makefile` target `backend-coverage`:
+
+- `app.core.error_handling`
+- `app.services.mentions`
+
+Command (CI):
+
+```bash
+cd backend && uv run pytest \
+  --cov=app.core.error_handling \
+  --cov=app.services.mentions \
+  --cov-branch \
+  --cov-report=term-missing \
+  --cov-report=xml:coverage.xml \
+  --cov-report=json:coverage.json \
+  --cov-fail-under=100
+```
+
+## Frontend scope (100% required)
+
+Enforced in `frontend/vitest.config.ts` coverage settings:
+
+- include: `src/lib/backoff.ts`
+- thresholds: 100% for lines/statements/functions/branches
+
+This is intentionally limited to a single pure utility module first. As we add more unit tests in `src/lib/**` and React Testing Library component tests for `src/app/**` + `src/components/**`, we should expand the include list and keep thresholds strict.
+
+## How to expand the gate
+
+- Add tests for the next-highest-signal modules.
+- Add them to the gated scope (backend `--cov=` list; frontend `coverage.include`).
+- Keep the threshold at 100% for anything included in the gate.