refactor: ColumnSpec / AddColumn carry constraint fields (ADR-0029 scaffolding)

Expand ColumnSpec and Command::AddColumn with the four
ADR-0029 constraint slots (not_null, unique, default, check),
all defaulting off; `Database::add_column` now takes a
ColumnSpec. No behaviour change — the grammar to set the
fields and the DDL to enforce them land in the following
commits. Isolated here so those commits stay readable.

Adds ColumnSpec::new for the unconstrained case; 110 call
sites updated. 1172 tests pass; clippy clean.

src/app.rs

@@ -2209,10 +2209,7 @@ mod tests {
             command,
             &Command::CreateTable {
                 name: "Customers".to_string(),
-                columns: vec![crate::dsl::ColumnSpec {
-                    name: "id".to_string(),
-                    ty: Type::Serial,
-                }],
+                columns: vec![crate::dsl::ColumnSpec::new("id", Type::Serial)],
                 primary_key: vec!["id".to_string()],
             },
         );
@@ -2419,10 +2416,7 @@ mod tests {
         let mut app = App::new();
         let cmd = Command::CreateTable {
             name: "Customers".to_string(),
-            columns: vec![crate::dsl::ColumnSpec {
-                name: "id".to_string(),
-                ty: Type::Serial,
-            }],
+            columns: vec![crate::dsl::ColumnSpec::new("id", Type::Serial)],
             primary_key: vec!["id".to_string()],
         };
         let desc = sample_description("Customers");
@@ -3174,6 +3168,10 @@ mod tests {
                 table: "T".to_string(),
                 column: "Name".to_string(),
                 ty: Type::Text,
+                not_null: false,
+                unique: false,
+                default: None,
+                check: None,
             },
         );
     }