[test-qa-hourly] test: cover impressions reach fallback

[i-xtsu-sixyou-ken-mei]

QA scope

Adds one focused regression test for social follower attribution when a provider has no views value but does have cumulative impressions. The existing implementation intentionally falls back from views to impressions for reach; this test pins that behavior so a future refactor cannot silently turn valid X-style reach telemetry into null attribution.

Source-of-truth inspection

• Recent main: latest checked commit e7a1edb593ee6824ad76fb3feb1ad63787832d86 (docs: reconcile Control Center operational truth (#436)), immediately after merged hourly QA PR [test-qa-hourly] test: classify missing canonical jobs as unknown #435.
• Reusable QA PR: none open. Previous QA PR [test-qa-hourly] test: classify missing canonical jobs as unknown #435 is merged, so this run created test-qa/ongoing-coverage-20260910 from current main.
• Open PRs: feat(control-center): add AI-readable incident correlation context #437, refactor(control-center): redesign around solo operator decisions #438, Codex/integrate 437 438 #439. Codex/integrate 437 438 #439 is a broad integration PR spanning Control Center/ops/podcast/migration changes, so QA work was intentionally kept separate.
• Active PR diff inspection: reviewed Codex/integrate 437 438 #439 changed-file scope; no overlap with this new test file.
• CI workflow: .github/workflows/ci.yml still runs quick gates (format repo contracts), code quality (type-check lint deadcode dup), tests (test analytics), plus E2E/security/deploy gates.
• Coverage: scripts/coverage.sh still runs turbo run test:coverage followed by tsx scripts/coverage-summary.ts for the summary command.
• Relevant implementation: computePostActivity() uses views ?? impressions for reach and clamps cumulative counter deltas.
• Relevant tests: existing social-attribution.test.ts covers baseline/endpoints, resets, unavailable observations, virtual-zero posts, weighting, churn, and YouTube exact attribution, but did not directly pin impressions fallback.

Change

• Added apps/control-center/src/server/services/social-attribution-impressions.test.ts.
• No production code, workflow, docs, thresholds, mocks, ignores, or unrelated refactors changed.

Commands / validation

• GitHub source-of-truth inspection — PASS.
• Local focused validation attempt via the authenticated DevSpace connector — BLOCKED by transient account connection error (We couldn't connect your account). No local repository files were modified.
• PR CI — pending; authoritative executable validation for this head.

Next QA target

After CI is green, inspect standardized 24h growth metrics for collection_status='unavailable': metrics24hForPostIds() currently filters post/window but not collection status, while the standardized query also does not filter collected rows. Verify whether unavailable rows can inflate samples/reach/engagement before making any minimal consumer/query fix.