feat: add user display name field (#898)

Co-authored-by: Elias Schneider <login@eliasschneider.com>

backend/resources/migrations/sqlite/20250829120000_user_display_name.down.sql

@@ -0,0 +1,3 @@
+BEGIN;
+ALTER TABLE users DROP COLUMN display_name;
+COMMIT;

backend/resources/migrations/sqlite/20250829120000_user_display_name.up.sql

@@ -0,0 +1,42 @@
+PRAGMA foreign_keys = OFF;
+BEGIN;
+
+CREATE TABLE users_new
+(
+    id           TEXT    NOT NULL PRIMARY KEY,
+    created_at   DATETIME,
+    username     TEXT    NOT NULL UNIQUE,
+    email        TEXT    NOT NULL UNIQUE,
+    first_name   TEXT,
+    last_name    TEXT    NOT NULL,
+    display_name TEXT    NOT NULL,
+    is_admin     NUMERIC NOT NULL DEFAULT FALSE,
+    ldap_id      TEXT,
+    locale       TEXT,
+    disabled     NUMERIC NOT NULL DEFAULT FALSE
+);
+
+INSERT INTO users_new (id, created_at, username, email, first_name, last_name, display_name, is_admin, ldap_id, locale,
+                       disabled)
+SELECT id,
+       created_at,
+       username,
+       email,
+       first_name,
+       COALESCE(last_name, ''),
+       TRIM(COALESCE(first_name, '') || ' ' || COALESCE(last_name, '')),
+       is_admin,
+       ldap_id,
+       locale,
+       disabled
+FROM users;
+
+DROP TABLE users;
+
+ALTER TABLE users_new
+    RENAME TO users;
+
+CREATE UNIQUE INDEX users_ldap_id ON users (ldap_id);
+
+COMMIT;
+PRAGMA foreign_keys = ON;