Add optional anonymous telemetry for usage insights

I’d like to suggest adding an optional, anonymous telemetry system to help the maintainers understand how LibreSign is used in real deployments.

The goal would be to collect only aggregate, privacy-friendly metrics such as:

• app / Nextcloud / PHP versions
• total user count
• active users in the last 30 days
• feature usage counters
• counts of completed / cancelled signing flows
• basic server environment info like database type and web server
• an anonymous instance hash generated locally, only to distinguish installations

Privacy should be a hard requirement:

• no personal data
• no document contents
• no file names
• no email addresses
• no user IDs
• no raw instance URL
• no secrets or tokens

Suggested approach:

• add a backend telemetry service to build and send the payload
• run it periodically via a background job
• add an admin setting to enable / disable telemetry
• add a manual “send report now” action for admins
• make failures silent so telemetry never affects normal app usage

This would help maintainers better understand adoption, feature usage, and the impact of releases while keeping the data collection minimal and
anonymous.

If there is interest, I’d be happy to help refine the metric list and implementation details.

The issue does not name implementation files, tests, or existing entry points. Start by reviewing the app's background-job and admin-settings infrastructure, then clarify the telemetry scope, privacy model, and reporting endpoint before implementation. Done should include opt-in controls, anonymous aggregate reporting, scheduled and manual submission, and failure isolation.