docs: update notification channel and API docs for form-based management
- NOTIFICATIONS.md: document owner/private fields, channel visibility
rules, and user-created channels; add troubleshooting note for
private channel visibility
- HTTP_API.md: add notification channel API endpoints table and full
endpoint reference (GET types, GET/POST/PUT/DELETE channels)
- USERS.md: add missing PUT /api/0/users/me endpoint documentation
with all three update modes (identity, channels, password)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Andreas Wrede
12e8812070