feat(cortex-app-server): implement backend API features from AGENT_10 (#209)

Implements the backend API features as specified in ORCHESTRATE/AGENT_10_BACKEND.md:

## New Features

### Session Sharing API
- POST /api/v1/share - Create a share link for a session with expiration and view limits
- GET /api/v1/share/:token - Retrieve a shared session (increments view count)
- DELETE /api/v1/share/:token - Revoke a share link
- GET /api/v1/share/:token/stats - Get share statistics

### Enhanced Agents/Droids API
- GET /api/v1/agents/builtin - List built-in agents (general, explore, research)
- POST /api/v1/agents/import - Import agent from markdown content
- PUT /api/v1/agents/:name - Update an existing agent

### Admin Sessions API
- GET /api/v1/admin/stats - Overall server statistics
- GET /api/v1/admin/stats/sessions - Session statistics
- GET /api/v1/admin/stats/usage - Usage statistics over time
- GET /api/v1/admin/sessions - List sessions with filtering/pagination
- POST /api/v1/admin/sessions/bulk - Bulk actions (delete)
- GET /api/v1/admin/sessions/export - Export sessions as CSV
- GET /api/v1/admin/shares - List all shares
- POST /api/v1/admin/shares/cleanup - Cleanup expired shares

### Task Progress WebSocket
- New TaskManager for real-time todo list tracking
- Task events broadcast to connected clients
- Support for TodoWrite format parsing

## Technical Details
- Added share.rs, admin.rs, tasks.rs modules
- Added ShareManager and TaskManager to AppState
- Added Gone (410) error type for expired resources
- Background cleanup of expired shares
- All new tests pass (40 total tests)

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt