fix: map default CEFR courses to Beginner/Intermediate/Advanced programs

Seed A1-A2, B1-B2, and C1-C2 only on their matching programs; add migration
000050 for existing databases. Document mapping in domain.

Made-with: Cursor

db/migrations/000048_seed_default_courses.up.sql

@@ -1,5 +1,5 @@
--- Default CEFR-style course names per program (custom courses can still be created via the API with any name).
--- Matches hierarchy note on courses: CEFR labels A1..C2, plus ad-hoc names allowed.
+-- Default CEFR-style courses per seeded program: Beginner→A1,A2; Intermediate→B1,B2; Advanced→C1,C2.
+-- Custom courses can still be created via the API with any name.
 INSERT INTO courses (program_id, name, description, thumbnail)
 SELECT
     p.id,
@@ -7,12 +7,13 @@ SELECT
     'Default CEFR level course (system seed).',
     NULL
 FROM programs AS p
-CROSS JOIN (
+INNER JOIN (
     VALUES
-        ('A1'),
-        ('A2'),
-        ('B1'),
-        ('B2'),
-        ('C1'),
-        ('C2')
-) AS v (name);
+        ('Beginner', 'A1'),
+        ('Beginner', 'A2'),
+        ('Intermediate', 'B1'),
+        ('Intermediate', 'B2'),
+        ('Advanced', 'C1'),
+        ('Advanced', 'C2')
+) AS v (program_name, name)
+    ON p.name = v.program_name;

db/migrations/000050_default_courses_per_program.down.sql

@@ -0,0 +1 @@
+-- Data cleanup is not reversed; restoring the old cross-product seed would be ambiguous.

db/migrations/000050_default_courses_per_program.up.sql

@@ -0,0 +1,45 @@
+-- Align default seeded courses with program: Beginner→A1,A2; Intermediate→B1,B2; Advanced→C1,C2.
+-- Only touches rows with the system seed description; custom courses are unchanged.
+-- Removing a course cascades to modules, lessons, and related LMS progress (see FKs on those tables).
+
+DELETE FROM courses AS c
+    USING programs AS p
+WHERE c.program_id = p.id
+  AND c.description = 'Default CEFR level course (system seed).'
+  AND (
+    (
+        p.name = 'Beginner'
+        AND c.name IN ('B1', 'B2', 'C1', 'C2')
+    )
+    OR (
+        p.name = 'Intermediate'
+        AND c.name IN ('A1', 'A2', 'C1', 'C2')
+    )
+    OR (
+        p.name = 'Advanced'
+        AND c.name IN ('A1', 'A2', 'B1', 'B2')
+    )
+  );
+
+INSERT INTO courses (program_id, name, description, thumbnail)
+SELECT
+    p.id,
+    v.name,
+    'Default CEFR level course (system seed).',
+    NULL
+FROM programs AS p
+INNER JOIN (
+    VALUES
+        ('Beginner', 'A1'),
+        ('Beginner', 'A2'),
+        ('Intermediate', 'B1'),
+        ('Intermediate', 'B2'),
+        ('Advanced', 'C1'),
+        ('Advanced', 'C2')
+) AS v (program_name, name)
+    ON p.name = v.program_name
+WHERE
+    NOT EXISTS (
+        SELECT 1 FROM courses AS e
+        WHERE e.program_id = p.id AND e.name = v.name
+    );

internal/domain/course.go

@@ -2,8 +2,15 @@ package domain
 
 import "time"
 
-// DefaultCEFRCourseNames are the standard course names seeded for each program (migration 000048).
-// Creating a course via the API may use any of these or a custom name.
+// DefaultCEFRCoursesByProgramName maps seeded program names to default course names
+// (migrations 000048 and 000050). The API may still create courses with any name.
+var DefaultCEFRCoursesByProgramName = map[string][]string{
+	"Beginner":     {"A1", "A2"},
+	"Intermediate": {"B1", "B2"},
+	"Advanced":     {"C1", "C2"},
+}
+
+// DefaultCEFRCourseNames is every CEFR label used in DefaultCEFRCoursesByProgramName.
 var DefaultCEFRCourseNames = []string{"A1", "A2", "B1", "B2", "C1", "C2"}
 
 // Course belongs to a Program.