Add custom interval days support for task frequency

Backend changes:
- Add "Custom" frequency option to database seeds
- Add custom_interval_days field to Task model
- Update task DTOs to accept custom_interval_days
- Update task service to use custom_interval_days for next due date calculation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

seeds/001_lookups.sql

@@ -65,7 +65,8 @@ VALUES
     (5, NOW(), NOW(), 'Monthly', 30, 5),
     (6, NOW(), NOW(), 'Quarterly', 90, 6),
     (7, NOW(), NOW(), 'Semi-Annually', 180, 7),
-    (8, NOW(), NOW(), 'Annually', 365, 8)
+    (8, NOW(), NOW(), 'Annually', 365, 8),
+    (9, NOW(), NOW(), 'Custom', NULL, 9)
 ON CONFLICT (id) DO UPDATE SET
     name = EXCLUDED.name,
     days = EXCLUDED.days,