…ty alerts (Priyanshu-byte-coder#2525)

## Summary

Implements GitHub Issue Priyanshu-byte-coder#2525 - Outbound Webhook Integrations for DevTrack
Activity Alerts.

## Changes

### New Events (Issue Priyanshu-byte-coder#2525 activity alerts)
- streak.milestone_reached - fired on streak milestone (7, 30, 100 days, etc.)
- goal.completed           - fired when a weekly coding goal is completed
- weekly_summary.ready     - fired when the weekly coding summary is generated

These are added alongside the existing 6 events and listed first in the UI.

### src/lib/webhooks.ts
- Add ACTIVITY_ALERT_EVENTS constant with the three new event types
- Add ActivityAlertEvent type
- Add isActivityAlertEvent() validator
- Add getActivityAlertEvents() helper
- Add dispatchActivityAlert() async helper (fire-and-forget, error-isolated)
- Fix Content-Type header to use per-webhook content_type column (defaults to
  application/json)
- Add X-DevTrack-Version: 1 header to all outbound webhook requests
- Fix misaligned indentation of redirect-check block

### src/app/api/webhooks/activity-alerts/route.ts (NEW)
- POST: receives activity-alert trigger events from server-side callers
  (goal-sync, cron jobs, streak checks); validates event type and dispatches
  asynchronously via dispatchActivityAlert()
- GET: returns the list of valid activity-alert event types (for discovery)
- Requires authenticated session; rejects non-activity-alert events

### src/app/api/webhooks/custom/route.ts
- Accept content_type field (stored in DB, used when dispatching)
- Accept optional user-supplied secret_token (falls back to auto-generated)
- Add streak.milestone_reached and weekly_summary.ready to valid events

### src/app/api/webhooks/custom/[id]/route.ts
- Add streak.milestone_reached and weekly_summary.ready to valid events
  in the PATCH handler

### src/components/webhook/WebhookManager.tsx
- Rewrite: Activity Alert events (Issue Priyanshu-byte-coder#2525) listed first with descriptions
- Add Content-Type selector (application/json / x-www-form-urlencoded)
- Add Secret Token input field with auto-generate fallback explanation
- Add copy-to-clipboard button for revealed secret tokens
- Rename heading to 'Outbound Webhooks' with clearer description
- Group events into 'Activity Alerts' and 'Other Events' sections

### test/webhook-activity-alerts.test.ts (NEW - 36 tests)
- ACTIVITY_ALERT_EVENTS constant validation
- isActivityAlertEvent() - all three events, negatives, null/undefined, case
- isValidWebhookEvent() - verifies new events accepted
- getActivityAlertEvents() - list consistency
- getAvailableEvents() - backwards-compat + new events present
- signPayload() - determinism, uniqueness per payload/secret
- dispatchActivityAlert() - no-op when no webhooks, supabase lookup, error
  isolation (DB failure does not throw), payload data injection
- GET /api/webhooks/activity-alerts - returns event list

### test/webhooks.test.ts
- Update event-count assertions from 6 to 8 (two new events added)