Add indoor_sowing_weeks, direct_sowing_weeks, transplanting_weeks,
glasshouse_weeks, harvesting_weeks (integer[]) to backend Cultivar and
CreateCultivar structs with INSERT/UPDATE SQL bindings.
Frontend PlantingCalendar component rewritten as a compact 52-column
Gantt-style grid grouped by month headers. Prefers week data when
available, falls back to expanding month data into week ranges.
Calendar displayed full-width on cultivar detail page with color legend.
florian.berthold
170aa84a0f