docs: Document the unique(per...) index shorthand

docs/06-concepts/02-models/01-models.md

@@ -547,7 +547,7 @@ fields:
 | [**type (index)**](database/indexing)                               | The type of index to create.                                                                                  |       ✅        |                         |               |
 | [**parameters (index)**](database/indexing#vector-indexes)          | Parameters for specialized index types like HNSW and IVFFLAT vector indexes.                                  |       ✅        |                         |               |
 | [**distanceFunction (index)**](database/indexing#vector-indexes)    | Distance function for vector indexes. Allowed values depend on the field's vector type and index type — see [vector indexes](database/indexing#vector-indexes). |       ✅        |                         |               |
-| [**unique**](database/indexing)                                     | Boolean flag to make the entries unique in the database.                                                      |       ✅        |                         |               |
+| [**unique**](database/indexing)                                     | Creates a unique index. |       ✅        |                         |               |
 | [**default**](#default-values)                                      | Sets the default value for both the model and the database. This keyword cannot be used with **relation**.    |       ✅        |                         |               |
 | [**defaultModel**](#default-values)                                 | Sets the default value for the model side. This keyword cannot be used with **relation**.                     |       ✅        |                         |               |
 | [**defaultPersist**](#default-values)                               | Sets the default value for the database side. This keyword cannot be used with **relation** and **!persist**. |       ✅        |                         |               |

docs/06-concepts/06-database/04-indexing.md

@@ -59,6 +59,46 @@ fields:
   name: String, unique
 ```
 
+### Composite unique constraints
+
+When a value should be unique only within a scope (for example, a setting key that is unique per user), use `unique(per=...)` on the field that must be unique within that scope. Serverpod auto-generates a composite unique index with the `per` columns first, followed by the annotated field.
+
+```yaml
+class: UserSetting
+table: user_setting
+fields:
+  userId: int
+  key: String, unique(per=userId)
+```
+
+In this example, two rows can share the same `key` if they belong to different users, but the same user cannot have two rows with the same `key`.
+
+For a scope that spans multiple columns, pass a list of field names:
+
+```yaml
+class: Product
+table: product
+fields:
+  tenantId: int
+  category: String
+  sku: String, unique(per=[tenantId, category])
+```
+
+You can also use the equivalent expanded form, where you name the index yourself:
+
+```yaml
+class: Product
+table: product
+fields:
+  tenantId: int
+  category: String
+  sku: String
+indexes:
+  product_unique_idx:
+    fields: tenantId, category, sku
+    unique: true
+```
+
 ## Specifying index type
 
 It is possible to add a type key to specify the index type.