feat: implement task dependencies with blocking/blocked-by relationships (#3185)

[knoxiboy]

Summary

This PR implements task dependencies with blocking/blocked-by relationships for the Kanban board. It supports adding/removing dependencies, displaying visual indicators on cards, and enforcing a drag-and-drop rule preventing blocked tasks from being moved to the final "Done" column until all blocking tasks are completed.

Closes #3185

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Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• Database / Schema:
  • supabase/migrations/20260715000002_add_task_dependencies.sql: Created task_dependencies join table with RLS policies, indexing, self-dependency prevention check, and a unique pair constraint.
• Backend / API:
  • src/app/api/kanban/[projectId]/route.ts: Updated GET query to fetch all task dependency mappings.
  • src/app/api/kanban/[projectId]/tasks/[taskId]/dependencies/route.ts: Created endpoints to POST (create) and DELETE (remove) task dependencies.
• Frontend / UI Components:
  • src/components/kanban/DependencyModal.tsx: Visual modal allowing users to manage task blocking relationships.
  • src/components/kanban/KanbanTaskCard.tsx: Render red "Blocked By" and indigo "Blocking" badges showing dependency statistics.
  • src/components/kanban/KanbanColumn.tsx: Calculate and propagate dependency blocker counts to individual cards.
  • src/components/kanban/KanbanBoard.tsx: Integrated drag-and-drop move verification to block dropping tasks onto the final stage if they are blocked by unfinished tasks.

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How to Test

1. Navigate to a Kanban board page.
2. Create two tasks: "Write Documentation" and "Deploy Production".
3. Click the dependency link button (or click a badge) on "Deploy Production".
4. Set "Deploy Production" to be blocked by "Write Documentation" and save.
5. Attempt to drag "Deploy Production" to the final "Done" column. Verify that an alert listing the blocker appears and the card snaps back.
6. Move "Write Documentation" to "Done".
7. Drag "Deploy Production" to "Done". Verify that it is now allowed.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally (Eslint setup failed locally on native dependency issue)
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed
• Tested on mobile / responsive layout

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

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• level:critical — 80 pts

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