Canteen Management

Adds a "Todays production (optional)" section to the recipe dialog -- pick a meal slot, type how many portions (e.g. 100 idli today), and it writes straight into ProductionPlanLine.CanteenForecastPortions for today -- the Canteen (fcst) column on the Production and wastage pages cook sheet, which has read 0 since that page was built (the automated same-weekday forecast, F4-1, was never built; this is a manual substitute for it, not an attempt at the real thing).

New Recipe.SetCanteenForecast / ProductionPlanLine.SetCanteenForecast on the entity, a new IProductionPlanService.SetCanteenForecastAsync that creates todays plan line if a canteen-only recipe has none yet (e.g. a counter item with no patient demand) or updates the existing one without touching its patient portions, and POST /api/canteen/production/forecast (canteen.production.update). No migration needed -- CanteenForecastPortions already existed as a column, just always 0.

Updated the Production pages own stat card and explanatory text, which still said "not built yet" and "reads 0 rather than invent a number" -- now accurate again.

Verified live: set Dhosha to 100 for Lunch from the Recipes dialog, confirmed it appears on the real cook sheet (Production and wastage page) under Lunch, Canteen (fcst) 100, Total to cook 100, 1 batch of 100. Full backend suite green (2 pre-existing unrelated coupon failures only; also fixed 5 unrelated DietOrderPollJobTests failures that surfaced from swapping ProductionPlanServices unused TimeProvider dependency for IBusinessClock, needed to compute todays date correctly). Full frontend suite green.