Add sprints, milestones, analytics, dashboard, and user management

[thanhtoan226-crypto]

Summary

• Sprint management: Create/manage sprints with status workflows (planning/active/completed), assign cards to sprints
• Milestone tracking: Track milestones with statuses (upcoming/in_progress/completed), due dates, and board-level visibility
• Board analytics panel: Recharts-powered visualizations — column distribution, priority breakdown, velocity line chart, completion rate, story points tracking
• Dashboard page: Overview with stats cards, recently active boards, overdue cards, and quick navigation
• Board settings: WIP limits, default card type, and description per board
• User management panel: Admin user listing with search and stats

Backend changes

• New DB modules: board_settings.py, milestone.py, sprint.py, user.py
• New API routes: /api/milestones/*, /api/sprints/*
• Schema additions: milestones, sprints tables; board settings columns (wip_limit, default_card_type, description)
• Enhanced card model with sprint assignment support
• Auth route extended with user listing endpoint

Frontend changes

• New components: DashboardPage, SprintSelector, MilestoneTracker, BoardAnalyticsPanel, BoardSettingsPanel, UserManagementPanel
• New shared API client (api.ts) with CSRF header support
• Enhanced CardDetailModal, KanbanBoard, SearchFilter, BoardList
• Added recharts dependency for analytics charts
• Test files for all new components

Test plan

• Create/edit/delete sprints and assign cards
• Create/edit/complete milestones with due dates
• Verify analytics charts render with board data
• Dashboard shows correct stats and overdue cards
• Board settings persist WIP limits and default card type
• User management panel lists and searches users
• Existing Kanban board workflow unaffected

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