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/household.dbml

@@ -0,0 +1,31 @@
+Table household_items {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  category text
+  location text
+  details jsonb [not null, default: `'{}'`]
+  notes text
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    category
+  }
+}
+
+Table household_vendors {
+  id uuid [pk, default: `gen_random_uuid()`]
+  name text [not null]
+  service_type text
+  phone text
+  email text
+  website text
+  notes text
+  rating int
+  last_used date
+  created_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    service_type
+  }
+}