HOTFIX web: /status/full 500 (str→tzinfo), /plan/compare 500 (chybějící comparison), canonical PV fn 4.5s→0.4s (force_custom_plan)

1) full_status._iso_utc dostával z JSONB bundle stringy → AttributeError → 500
   celého /status/full; nyní parsuje přes _parse_ts.
2) /plan/compare: NameError — 'comparison = _bundle_from_current(compare_raw)'
   se nikdy nesestavilo (smazaný řádek), endpoint vždy 500.
3) fn_forecast_pv_slots_range_canonical_ab: PG 18 cachuje plány SQL funkcí →
   generický plán 4.5 s / 607k buffers; set plan_cache_mode=force_custom_plan
   → 0.4 s / 34k (změřeno explain analyze na živé DB). Táhne /plan/current,
   /plan/compare i rolling plánovač.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/routers/plan.py

@@ -207,6 +207,7 @@ async def get_plan_compare(
         raise HTTPException(status_code=404, detail="No comparison plan")
 
     active = _bundle_from_current(active_raw)
+    comparison = _bundle_from_current(compare_raw)
     diff, slot_diffs = _build_plan_diff(active, comparison)
     return PlanningCompareResponseModel(
         active=active,