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>

tests/test_admin.py

@@ -600,6 +600,7 @@ def test_source_image_policy(tmp_path, monkeypatch):
     c = sqlite3.connect(os.environ["GOODNEWS_DB"])
     assert c.execute("SELECT image_policy FROM sources WHERE id=2").fetchone()[0] == "cache"
     c.close()
-    assert tc.post("/api/admin/sources/2/image-policy", json={"policy": "remote"}).json()["policy"] == "remote"
+    r = tc.post("/api/admin/sources/2/image-policy", json={"policy": "remote"}).json()
+    assert r["policy"] == "remote" and r["purged"] == 0    # leaving cache purges (no files here)
     assert tc.post("/api/admin/sources/2/image-policy", json={"policy": "bogus"}).status_code == 422
     assert tc.post("/api/admin/sources/999/image-policy", json={"policy": "cache"}).status_code == 404