feat: enable Fischerei, Sitzungsprotokolle, Verbandsverwaltung modules

- Enable all 3 modules via NEXT_PUBLIC_ENABLE_* build args + runtime env
- Fix empty-string-to-null for date/optional columns in all module APIs:
  fischerei (24 fixes), verbandsverwaltung (15 fixes), sitzungsprotokolle (2 fixes)
- CACHE_BUST=12 for full rebuild with new feature flags

packages/features/sitzungsprotokolle/src/server/api.ts

@@ -79,9 +79,9 @@ export function createMeetingsApi(client: SupabaseClient<Database>) {
         .insert({
           account_id: input.accountId,
           title: input.title,
-          meeting_date: input.meetingDate,
+          meeting_date: input.meetingDate || null,
           meeting_type: input.meetingType,
-          location: input.location,
+          location: input.location || null,
           attendees: input.attendees,
           remarks: input.remarks,
           is_published: input.isPublished,