fix(dashboard_api): jsonable_encoder on /health/ingest (datetime serialize)

The ingest-health handler returned reporting.v_ingest_health rows straight to
JSONResponse, but last_run_at is a datetime — json.dumps raised TypeError and the
endpoint fell into its except, always returning {"overall":"unknown","endpoints":[]}.
Every other analytics endpoint already routes through jsonable_encoder; this one
didn't. Surfaced when the prod bridge finally got the /health/ingest route.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

dashboard_api_rev.py

@@ -205,7 +205,10 @@ def ingest_health():
             else "stale" if any(r["freshness"] == "stale" for r in rows)
             else "ok"
         ) if rows else "unknown"
-        return JSONResponse({"overall": worst, "endpoints": rows})
+        # rows carry last_run_at (datetime) — jsonable_encoder (Decimal→float,
+        # datetime→ISO) before JSONResponse, else json.dumps raises TypeError
+        # and the whole feed 500s into the except below. (260702 fix.)
+        return JSONResponse(jsonable_encoder({"overall": worst, "endpoints": rows}))
     except Exception:
         log.exception("ingest-health failed")
         return JSONResponse(