[Critical] Add pagination to feature endpoints to prevent performance issues

Problem

Currently, GET /api/features/[featureId] returns the entire feature with ALL phases and ALL tasks in a single response. This creates several issues:

• Unbounded response size: A feature with 200 tasks results in a multi-MB JSON response
• Browser performance: React rendering hundreds of task rows can freeze the UI
• Network timeouts: Large responses timeout on slow connections
• Scale ceiling: System will break at ~200 tasks per feature

Current Implementation

API Route: /src/app/api/features/[featureId]/route.ts

// Returns everything at once
const feature = await getFeature(featureId, userId);
// Includes: feature.phases[].tasks[] - unbounded arrays

Type Definition: /src/types/roadmap.tsx

export interface FeatureDetail {
  phases: PhaseWithTasks[];  // Each phase has tasks: TicketListItem[]
  // No pagination metadata
}

Frontend: /src/app/w/[slug]/roadmap/[featureId]/page.tsx

• Fetches entire feature on mount
• Passes all tasks to TicketsList component
• No virtualization or windowing

Proposed Solution

Lazy-load tasks when user navigates to Tasks tab

Phase 1: Separate endpoints

GET /api/features/[featureId]  // Feature + phases metadata only (no tasks)
GET /api/features/[featureId]/tasks?page=1&limit=50  // Paginated tasks

Phase 2: Frontend changes

• Feature detail page fetches feature without tasks
• Tasks tab triggers separate fetch when user clicks "Tasks"
• Add virtual scrolling with react-window for task list rendering

Phase 3: Backend optimization

• Add cursor-based pagination for efficient queries
• Return pagination metadata: { data: [], pagination: { total, page, limit, hasMore } }

Alternative: Query parameter approach

GET /api/features/[featureId]?includeTasks=false  // Default: exclude tasks
GET /api/features/[featureId]?includeTasks=true&tasksPage=1&tasksLimit=50

Success Criteria

• Feature with 500 tasks loads in <2 seconds
• Browser doesn't freeze when rendering task list
• API response size stays under 1MB regardless of task count
• Tasks are fetched only when user navigates to Tasks tab

Code References

• /src/app/api/features/[featureId]/route.ts - API route handler
• /src/services/roadmap/features.ts - getFeature service function
• /src/types/roadmap.tsx - FeatureDetail type (lines 620-640)
• /src/components/features/TicketsList/index.tsx - Frontend component that renders tasks

Effort Estimate

2-3 weeks

• Backend API changes: 3-4 days
• Frontend refactoring: 4-5 days
• Virtual scrolling implementation: 2-3 days
• Testing and optimization: 3-4 days

Priority

Critical - This will become a blocker as features grow beyond 50-100 tasks.

Start with src/app/api/features/[featureId]/route.ts and src/services/roadmap/features.ts to trace how phases and tasks are loaded, then review FeatureDetail in src/types/roadmap.tsx and TicketsList in src/components/features/TicketsList/index.tsx. Define the endpoint and loading changes needed for lazy, paginated tasks and verify the success criteria: bounded responses, no UI freeze, and tasks fetched only on the Tasks tab.