feat: add cumulative checkin tracking mode for goals

Goals can now choose between milestone-based progress (existing) and
cumulative checkin-based progress (new). Cumulative mode supports
cross-unit conversion (e.g. kcal → g fat) via a configurable
conversion rate. New GoalCheckin model stores daily inputs; progress
auto-recalculates on every checkin C/U/D. Backup import/export covers
the new table. Frontend GoalDialog, GoalDetailPage and GoalPage cards
adapt to show cumulative progress or milestone progress based on
track_type.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

api/app/models/__init__.py

@@ -4,7 +4,7 @@ from app.models.tag import Tag, task_tags
 from app.models.user_settings import UserSettings
 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.goal import Goal, GoalStep, GoalReview, GoalCheckin, goal_tasks
 from app.models.sync_settings import SyncSettings
 from app.models.certificate import Certificate, CertificateCategory
 
@@ -12,7 +12,7 @@ __all__ = [
     "Task", "Category", "Tag", "task_tags", "UserSettings",
     "HabitGroup", "Habit", "HabitCheckin",
     "AnniversaryCategory", "Anniversary",
-    "Goal", "GoalStep", "GoalReview", "goal_tasks",
+    "Goal", "GoalStep", "GoalReview", "GoalCheckin", "goal_tasks",
     "SyncSettings",
     "Certificate", "CertificateCategory",
 ]