images/analytics: purge on policy revoke + engagement warm-up note (Codex close-out)

- newsimg.purge_source(): when a source leaves 'cache' (permission revoked / re-classified),
  the admin image-policy endpoint now deletes that source's re-hosted copies immediately,
  rather than leaving them inaccessible-but-on-disk. Endpoint returns {purged}.
- Admin "Engaged readers" carries a warm-up note: tracking began 2026-06-30, so low
  rolling windows are partly warm-up, not all bots (compare d7 after a week, the window
  after its full span). Guards against misreading "6 engaged vs 135 visits" as 129 bots.
Tests: purge_source removes only the target source's copies; endpoint reports purged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/api.py

@@ -1437,7 +1437,11 @@ def create_app() -> FastAPI:
             if cur.rowcount == 0:
                 raise HTTPException(status_code=404, detail="source not found")
             conn.commit()
-        return {"ok": True, "policy": pol}
+            # Leaving 'cache' (e.g. permission revoked) → take the re-hosted copies down now,
+            # not just make them inaccessible. Setting TO 'cache' just flips the flag; the
+            # cycle warms it.
+            purged = newsimg.purge_source(conn, sid) if pol != "cache" else 0
+        return {"ok": True, "policy": pol, "purged": purged}
 
     # --- Source candidates (supervised add-a-source pipeline) ----------------