feat(sqlite-demo): add Post↔Tag M:N relation + emit

Adds Tag model, PostTag junction (composite PK post_tag_pkey), and
rel.manyToMany() declarations on both Post.tags and Tag.posts.
Re-emits contract.json / contract.d.ts; cardinality 'N:M' + through
descriptor present in the generated artifacts.

Signed-off-by: Alexey Orlenko's AI Agent <robot@aqrln.net>

Author: tensordreams

examples/prisma-next-demo-sqlite/prisma/contract.ts

@@ -20,6 +20,22 @@ export const contract = defineContract({}, ({ field, model }) => {
     },
   });
 
+  const Tag = model('Tag', {
+    fields: {
+      id: field.id.uuidv4(),
+      label: field.column(textColumn),
+    },
+  });
+
+  const PostTag = model('PostTag', {
+    fields: {
+      postId: field.uuid(),
+      tagId: field.uuid(),
+    },
+  }).attributes(({ fields, constraints }) => ({
+    id: constraints.id([fields.postId, fields.tagId], { name: 'post_tag_pkey' }),
+  }));
+
   return {
     models: {
       User: User.relations({
@@ -29,6 +45,11 @@ export const contract = defineContract({}, ({ field, model }) => {
       }),
       Post: Post.relations({
         user: rel.belongsTo(User, { from: 'userId', to: 'id' }),
+        tags: rel.manyToMany(() => Tag, {
+          through: () => PostTag,
+          from: 'postId',
+          to: 'tagId',
+        }),
       }).sql(({ cols, constraints }) => ({
         table: 'post',
         foreignKeys: [
@@ -37,6 +58,22 @@ export const contract = defineContract({}, ({ field, model }) => {
           }),
         ],
       })),
+      Tag: Tag.relations({
+        posts: rel.manyToMany(() => Post, {
+          through: () => PostTag,
+          from: 'tagId',
+          to: 'postId',
+        }),
+      }).sql({
+        table: 'tag',
+      }),
+      PostTag: PostTag.sql(({ cols, constraints }) => ({
+        table: 'post_tag',
+        foreignKeys: [
+          constraints.foreignKey(cols.postId, Post.refs.id, { name: 'post_tag_postId_fkey' }),
+          constraints.foreignKey(cols.tagId, Tag.refs.id, { name: 'post_tag_tagId_fkey' }),
+        ],
+      })),
     },
   };
 });