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/meals.dbml

@@ -0,0 +1,49 @@
+Table recipes {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  cuisine text
+  prep_time_minutes int
+  cook_time_minutes int
+  servings int
+  ingredients jsonb [not null, default: `'[]'`]
+  instructions jsonb [not null, default: `'[]'`]
+  tags "text[]" [not null, default: `'{}'`]
+  rating int
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    cuisine
+    tags
+  }
+}
+
+Table meal_plans {
+  id uuid [pk, default: `gen_random_uuid()`]
+  week_start date [not null]
+  day_of_week text [not null]
+  meal_type text [not null]
+  recipe_id uuid [ref: > recipes.id]
+  custom_meal text
+  servings int
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    week_start
+  }
+}
+
+Table shopping_lists {
+  id uuid [pk, default: `gen_random_uuid()`]
+  week_start date [unique, not null]
+  items jsonb [not null, default: `'[]'`]
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    week_start
+  }
+}