feat: Add generated and generated_always attributes

This commit introduces support for PostgreSQL generated columns by
adding two new field attributes to the `Georm` derive macro:
`#[georm(generated)]` and `#[georm(generated_always)]`.

The `#[georm(generated_always)]` attribute is for fields that are
always generated by the database, such as `GENERATED ALWAYS AS
IDENTITY` columns or columns with a `GENERATED ALWAYS AS (expression)
STORED` clause. These fields are now excluded from `INSERT` and
`UPDATE` statements, preventing accidental writes and ensuring data
integrity at compile time.

The `#[georm(generated)]` attribute is for fields that have a default
value generated by the database but can also be manually overridden,
such as `GENERATED BY DEFAULT AS IDENTITY` columns. These fields
behave similarly to `#[georm(defaultable)]` fields, allowing them to
be omitted from `INSERT` statements to use the database-generated
value.

For now, the behaviour is the same between `#[georm(generated)]` and
`#[georm(defaultable)]`, but the addition of the former now will be
useful for future features.

Key changes:
- Added `generated` and `generated_always` attributes to
  `GeormFieldAttributes`.
- Introduced `GeneratedType` enum in the IR to represent the different
  generation strategies.
- Modified the `create` and `update` query generation to exclude
  fields marked with `#[georm(generated_always)]`.
- Integrated `#[georm(generated)]` fields with the existing
  defaultable struct logic.
- Added validation to prevent conflicting attribute usage, namely
  `#[georm(generated)]` and `#[georm(generated_always)]` on the same
  field.

Implements #3

README.md

@@ -181,9 +181,13 @@ let user_role = UserRole::find(pool, &id).await?;
 
 **Note**: Relationships are not yet supported for entities with composite primary keys.
 
-### Defaultable Fields
+### Defaultable and Generated Fields
 
-For fields with database defaults or auto-generated values, use the `defaultable` attribute:
+Georm provides three attributes for handling fields with database-managed values:
+
+#### `#[georm(defaultable)]` - Optional Override Fields
+
+For fields with database defaults that can be manually overridden:
 
 ```rust
 #[derive(Georm)]
@@ -200,22 +204,74 @@ pub struct Post {
 }
 ```
 
-This generates a `PostDefault` struct for easier creation:
+#### `#[georm(generated)]` - Generated by Default
+
+For PostgreSQL `GENERATED BY DEFAULT` columns that can be overridden but are typically auto-generated:
+
+```rust
+#[derive(Georm)]
+#[georm(table = "products")]
+pub struct Product {
+    #[georm(id, generated_always)]
+    pub id: i32,                    // GENERATED ALWAYS AS IDENTITY
+    #[georm(generated)]
+    pub sku_number: i32,            // GENERATED BY DEFAULT AS IDENTITY
+    pub name: String,
+    pub price: sqlx::types::BigDecimal,
+}
+```
+
+#### `#[georm(generated_always)]` - Always Generated
+
+For PostgreSQL `GENERATED ALWAYS` columns that are strictly managed by the database:
+
+```rust
+#[derive(Georm)]
+#[georm(table = "products")]
+pub struct Product {
+    #[georm(id, generated_always)]
+    pub id: i32,                    // GENERATED ALWAYS AS IDENTITY
+    pub name: String,
+    pub price: sqlx::types::BigDecimal,
+    pub discount_percent: i32,
+    #[georm(generated_always)]
+    pub final_price: Option<sqlx::types::BigDecimal>, // GENERATED ALWAYS AS (expression) STORED
+}
+```
+
+#### Generated Structs and Behavior
+
+Both `defaultable` and `generated` fields create a companion `<Entity>Default` struct:
 
 ```rust
 use georm::Defaultable;
 
-let post_default = PostDefault {
-    id: None,                       // Let database auto-generate
-    title: "My Post".to_string(),
-    published: None,                // Use database default
-    created_at: None,               // Use database default (NOW())
-    author_id: 42,
+let product_default = ProductDefault {
+    name: "Laptop".to_string(),
+    price: BigDecimal::from(999),
+    discount_percent: 10,
+    sku_number: None,               // Let database auto-generate
+    // Note: generated_always fields are excluded from this struct
 };
 
-let created_post = post_default.create(pool).await?;
+let created_product = product_default.create(pool).await?;
 ```
 
+#### Key Differences
+
+| Attribute          | INSERT Behavior                    | UPDATE Behavior | Use Case                                     |
+|--------------------|------------------------------------|-----------------|----------------------------------------------|
+| `defaultable`      | Optional (can override defaults)   | Included        | Fields with database defaults                |
+| `generated`        | Optional (can override generation) | Included        | `GENERATED BY DEFAULT` columns               |
+| `generated_always` | **Excluded** (always generated)    | **Excluded**    | `GENERATED ALWAYS` columns, computed columns |
+
+#### Important Notes
+
+- **`generated_always` fields are completely excluded** from INSERT and UPDATE statements to prevent database errors
+- **`generated` and `generated_always` cannot be used together** on the same field
+- **`generated` fields behave like `defaultable` fields** but are semantically distinct for future enhancements
+- **Option types cannot be marked as `defaultable`** to prevent `Option<Option<T>>` situations
+
 ### Relationships
 
 Georm supports comprehensive relationship modeling with two approaches: field-level relationships for foreign keys and struct-level relationships for reverse lookups.
@@ -512,6 +568,8 @@ post_default.create(pool).await?;
 ```rust
 #[georm(id)]                               // Mark as primary key
 #[georm(defaultable)]                      // Mark as defaultable field
+#[georm(generated)]                        // Mark as generated by default field
+#[georm(generated_always)]                 // Mark as always generated field
 #[georm(relation = { /* ... */ })]         // Define relationship
 ```