Recipe can now be scheduled

backend/sql/version_1.sql

@@ -1,3 +1,6 @@
+-- Datetimes are stored as 'ISO 8601' text format.
+-- For example: '2025-01-07T10:41:05.697884837+00:00'.
+
 -- Version 1 is the initial structure.
 CREATE TABLE [Version] (
     [id] INTEGER PRIMARY KEY,
@@ -165,9 +168,11 @@ CREATE TABLE [RecipeScheduled] (
     [id] INTEGER PRIMARY KEY,
     [user_id] INTEGER NOT NULL,
     [recipe_id] INTEGER NOT NULL,
-    [date] TEXT NOT NULL,
+    [date] TEXT NOT NULL, -- In form of 'YYYY-MM-DD'.
     [servings] INTEGER, -- If NULL use [recipe].[servings].
 
+    UNIQUE([user_id], [recipe_id], [date]),
+
     FOREIGN KEY([user_id]) REFERENCES [User]([id]) ON DELETE CASCADE,
     FOREIGN KEY([recipe_id]) REFERENCES [Recipe]([id]) ON DELETE CASCADE
 );