feat: Webhook support for secret and environment change events

[mrkswrnr]

Description

Thank you for this great project!
Shelve already does a great job centralizing secrets and environment variables, but webhook support would make it much more useful in automated workflows and external integrations.

Problem

Right now, when secrets or environment configuration change, there does not seem to be an easy way to notify external systems automatically.

This makes it harder to build workflows such as:

• triggering deployments when a variable changes
• invalidating caches after secret rotation
• notifying internal tooling or chat systems
• syncing changes to custom infrastructure
• auditing and reacting to changes in real time

For a tool focused on secrets management, sync, and developer workflow, webhooks would be a very natural integration point.

Proposed solution

Add first-class webhook support so users can subscribe to events from Shelve and send signed HTTP POST requests to configured endpoints.

Example events

Useful events could include:

• secret.created
• secret.updated
• secret.deleted
• environment.created
• environment.updated
• environment.deleted
• project.updated
• member.invited
• member.removed

Expected behavior

• allow configuring one or more webhook endpoints per workspace, project, or environment
• allow selecting which event types should trigger delivery
• send JSON payloads with relevant metadata
• sign requests with a shared secret, for example via HMAC
• retry failed deliveries with backoff
• expose delivery history/logs for debugging

I would also be willing to implement this if you accept PRs

Additional context

No response