feat: add certificate management module with image upload

- Add Certificate + CertificateCategory models with full CRUD API
- Image upload via base64 data URL stored in Text column
- Certificate fields: title, issuer, issue_date, expiry_date, image, description
- Frontend: card grid with category sidebar filter, create/edit dialog
- Include certificates in data backup/export
- Fix hasPhaseParent optimization in GoalDetailPage

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/app/models/__init__.py

@@ -6,6 +6,7 @@ from app.models.habit import HabitGroup, Habit, HabitCheckin
 from app.models.anniversary import AnniversaryCategory, Anniversary
 from app.models.goal import Goal, GoalStep, GoalReview, goal_tasks
 from app.models.sync_settings import SyncSettings
+from app.models.certificate import Certificate, CertificateCategory
 
 __all__ = [
     "Task", "Category", "Tag", "task_tags", "UserSettings",
@@ -13,4 +14,5 @@ __all__ = [
     "AnniversaryCategory", "Anniversary",
     "Goal", "GoalStep", "GoalReview", "goal_tasks",
     "SyncSettings",
+    "Certificate", "CertificateCategory",
 ]