feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/calendar.dbml

@@ -0,0 +1,44 @@
+Table family_members {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  relationship text
+  birth_date date
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+}
+
+Table activities {
+  id uuid [pk, default: `gen_random_uuid()`]
+  family_member_id uuid [ref: > family_members.id]
+  title text [not null]
+  activity_type text
+  day_of_week text
+  start_time time
+  end_time time
+  start_date date
+  end_date date
+  location text
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    day_of_week
+    family_member_id
+    (start_date, end_date)
+  }
+}
+
+Table important_dates {
+  id uuid [pk, default: `gen_random_uuid()`]
+  family_member_id uuid [ref: > family_members.id]
+  title text [not null]
+  date_value date [not null]
+  recurring_yearly boolean [not null, default: false]
+  reminder_days_before int [not null, default: 7]
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    date_value
+  }
+}