feat: add the study data models and schema migrations

Courses, meetings, class sessions, deadlines, readings with per-user
progress, materials with tags and file attachments. Timestamps are
stored in UTC and held as Europe/Warsaw text so the admin text inputs
can write them.

models/nullable.go

@@ -0,0 +1,50 @@
+package models
+
+import "strings"
+
+// blank clears each optional text column whose value is empty or only
+// whitespace, so an emptied admin text input stores NULL rather than "".
+func blank(fields ...**string) {
+	for _, f := range fields {
+		if *f != nil && strings.TrimSpace(**f) == "" {
+			*f = nil
+		}
+	}
+}
+
+// temporalErrors is a field-to-messages map in the shape of lagoon.Validate.
+type temporalErrors map[string][]string
+
+func (e temporalErrors) add(field, msg string) { e[field] = append(e[field], msg) }
+
+func (e temporalErrors) dateTime(field string, v DateTime, required bool) {
+	switch {
+	case v == "" && required:
+		e.add(field, "The "+strings.ReplaceAll(field, "_", " ")+" field is required.")
+	case v != "" && !v.Valid():
+		e.add(field, "The "+strings.ReplaceAll(field, "_", " ")+" must be a date and time such as 2026-10-03 08:00.")
+	}
+}
+
+func (e temporalErrors) date(field string, v Date, required bool) {
+	switch {
+	case v == "" && required:
+		e.add(field, "The "+strings.ReplaceAll(field, "_", " ")+" field is required.")
+	case v != "" && !v.Valid():
+		e.add(field, "The "+strings.ReplaceAll(field, "_", " ")+" must be a date in the YYYY-MM-DD form.")
+	}
+}
+
+func (e temporalErrors) result() map[string][]string {
+	if len(e) == 0 {
+		return nil
+	}
+	return e
+}
+
+// TemporalValidator is implemented by models with Date or DateTime columns.
+// The admin controllers call it before a save, because lagoon.Validate has
+// no date rules.
+type TemporalValidator interface {
+	TemporalErrors() map[string][]string
+}