refactor: Address PR review feedback for timestamp handling

- Rename format-timestamp-for-postgres to format-timestamp-iso8601 (database-agnostic)
- Reuse format-timestamp-iso8601 in authenticate-user instead of inline formatting
- Change listened_at field type from integer to timestamp for consistency

database.lisp

@@ -61,7 +61,7 @@
     (db:create "user_listening_history" '((user_id :integer)
                                           (track_title :text)
                                           (track_artist :text)
-                                          (listened_at :integer)
+                                          (listened_at :timestamp)
                                           (duration_seconds :integer))))
 
   (unless (db:collection-exists-p "user_playlists")
@@ -98,8 +98,8 @@
   (string= (string-upcase (package-name (db:implementation)))
            "I-LAMBDALITE"))
 
-(defun format-timestamp-for-postgres (value)
-  "Convert a timestamp value to ISO 8601 format for PostgreSQL.
+(defun format-timestamp-iso8601 (value)
+  "Convert a timestamp value to ISO 8601 format.
 Handles: integers (Unix epoch), local-time timestamps, strings, and NIL."
   (cond
     ((null value) nil)
@@ -124,10 +124,10 @@ Handles: integers (Unix epoch), local-time timestamps, strings, and NIL."
           (last-login (dm:field data-model "last-login")))
       (when created-date
         (setf (dm:field data-model "created-date")
-              (format-timestamp-for-postgres created-date)))
+              (format-timestamp-iso8601 created-date)))
       (when last-login
         (setf (dm:field data-model "last-login")
-              (format-timestamp-for-postgres last-login))))))
+              (format-timestamp-iso8601 last-login))))))
 
 (defun data-model-save (data-model)
   "Wrapper on data-model save method to bypass error using dm:save on lambdalite.

user-management.lisp

@@ -70,14 +70,10 @@
         (when (and (= 1 user-active)
                    (verify-password password user-password))
           ;; Update last login using data-model (database agnostic)
-          ;; Use ISO 8601 format in UTC that PostgreSQL TIMESTAMP can parse
           (handler-case
               (progn
                 (setf (dm:field user "last-login") 
-                      (local-time:format-timestring nil (local-time:now)
-                                                    :format '(:year "-" (:month 2) "-" (:day 2) " " 
-                                                              (:hour 2) ":" (:min 2) ":" (:sec 2))
-                                                    :timezone local-time:+utc-zone+))
+                      (format-timestamp-iso8601 (local-time:now)))
                 ;; Use data-model-save to normalize all timestamp fields before saving
                 (data-model-save user))
             (error (e)