fix(knowledge): distinguish observations from review activity

[jomcgi]

The Monolith entity page could show an October 11 observation date after a later review because the entity response omitted last_reviewed_at. The daily chart was labelled "Facts written per day" while its query groups the whole public corpus by observed_at and current verification state.

Expose the existing public last_reviewed_at field, display review dates separately in the entity summary and each fact, and label the chart "Public facts by observation date". Keep original evidence dates, corpus filters, entity associations, ordering, review behavior and cache policy unchanged. The payload-derived ETag now changes when the displayed review timestamp changes.

Validation:

• 80 public-router tests passed. The new missing-review regression failed on the baseline with KeyError: last_reviewed_at, then passed. It also checks a conditional request returns the changed review without changing observed_at or indexed_at.
• 39 frontend tests passed across the rendered entity page, page loader, charts and public proxies.
• ci lint, Ruff, diff check and source ratchet passed.
• Independent whole-diff review found no blockers.
• Exact-head pr-checks passed at b3c7fb275024e4c1ec41da0425b63dee1ef76192: Executed 389 out of 460 tests, all 460 passed. Image builds, repository gates and browser checks passed; zero deployment-manifest changes across 17 applications. BuildBuddy run. The runner recovered automatically from missing remote-cache inputs; the HTTP smoke target passed its automatic retry after a healthz ReadTimeout. No manual rerun.

Investigation limits: the entity view is linked current public facts, subjects first, capped at sixty; the chart is all public facts in its observation-date window, including historical rows. Neither measures new writes or review events. Fresh MCP reads at 2026-10-13 16:04 UTC found 22 due KG jobs (17 unclaimed and five older locked rows); recent replica log samples did not establish current kg_full refusals. The public entity HTTP read returned 403 and was not retried or bypassed, so exact live entity membership, bar contributors and cache headers remain unverified. This patch addresses confirmed display semantics, not a claimed diagnosis of the entire live discrepancy.

Draft only. No merge, deployment, live setting, credential, budget, limit, permit release or recovery. Existing drafts #6973/#6963 and receipt 822/#6972 holds remain unchanged.

[jomcgi]

Helm Manifest Diff

No manifest changes detected across 17 application(s).