fix: make exercise goal and execution optional (at least one required)

Error: 224/225 exercises failed with NOT NULL constraint violations
Root cause: exercises table requires BOTH goal AND execution (NOT NULL)
Reality: Many wiki exercises have only ONE of these fields

Migration 019:
- ALTER COLUMN goal DROP NOT NULL
- ALTER COLUMN execution DROP NOT NULL
- ADD CHECK constraint: (goal IS NOT NULL OR execution IS NOT NULL)

This allows:
✓ Exercises with only goal
✓ Exercises with only execution
✓ Exercises with both
✗ Exercises with neither (CHECK constraint fails)

Import validation already correct (line 312):
required_ok = bool(mapped.get('goal') or mapped.get('execution'))

DB schema was blocking valid exercises.

backend/migrations/019_exercises_optional_fields.sql

@@ -0,0 +1,11 @@
+-- Migration 019: Make exercise goal and execution optional
+-- Many wiki exercises have only goal OR execution, not both
+
+ALTER TABLE exercises
+    ALTER COLUMN goal DROP NOT NULL,
+    ALTER COLUMN execution DROP NOT NULL;
+
+-- Add CHECK constraint: at least one of goal/execution must be present
+ALTER TABLE exercises
+    ADD CONSTRAINT exercises_goal_or_execution_required
+    CHECK (goal IS NOT NULL OR execution IS NOT NULL);