feat: add goal management module (long-term goals with phases, milestones, reviews)

Backend:
- Goal model: title, description, status (active/paused/completed/abandoned),
  progress (auto-computed from milestones), target_date, category, color, icon
- GoalStep model: unified phase/milestone with parent nesting
- GoalReview model: periodic reflection with rating
- goal_tasks M2M: link existing tasks to goals
- /api/goals CRUD + steps CRUD + reviews + task linking + status toggle
- Progress auto-calculated from milestone completion ratio

Frontend:
- GoalPage: card grid with progress bars, status filter
- GoalDetailPage: step tree (phases > milestones), reviews, linked tasks
- GoalDialog: create/edit form with color/icon picker
- Goal navigation in AppHeader
- useGoalStore: full Pinia store for all goal operations

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

api/app/database.py

@@ -59,7 +59,7 @@ def init_db():
     """初始化数据库表，自动补充新增的列"""
     # 导入所有模型，确保 Base.metadata 包含全部表定义
     from app.models import (  # noqa: F401
-        task, category, tag, user_settings, habit, anniversary,
+        task, category, tag, user_settings, habit, anniversary, goal,
     )
     Base.metadata.create_all(bind=engine)