feat(01-03): create Repository pattern with typed errors

- Add User domain model (summer_users table mapping)
- Add RepositoryError ADT (NotFound, Conflict, ValidationError, DatabaseError)
- Implement UserRepository trait with CRUD operations
- Add UserRepositoryLive using Quill with compile-time SQL validation
- Handle SQL exceptions with refineOrDie for typed error channel

summercms/src/model/User.scala

@@ -0,0 +1,26 @@
+package model
+
+import java.time.Instant
+
+/** User domain model
+  *
+  * Represents a registered user in the system. Maps to the summer_users table.
+  *
+  * @param id
+  *   Auto-generated primary key
+  * @param email
+  *   Unique email address, used for login
+  * @param passwordHash
+  *   Bcrypt-hashed password (never store plain text)
+  * @param createdAt
+  *   Timestamp when user was created
+  * @param updatedAt
+  *   Timestamp of last update
+  */
+case class User(
+    id: Long,
+    email: String,
+    passwordHash: String,
+    createdAt: Instant,
+    updatedAt: Instant
+)