feat: add Bitbucket VCS integration

[maartenbode]

Closes #24

Summary

• Implements Bitbucket Cloud as a first-party VCS provider using API v2.0 with App Password authentication (username + app_password)
• Adds BitbucketProvider with full support for tags, branches, file content, webhooks (HMAC-SHA256 via X-Hub-Signature), archive downloads, workspace listing, and repository browsing
• Adds Bitbucket webhook controller, signature verification middleware, and route
• Updates GitProviderInterface::deleteWebhook() signature from int to int|string to support Bitbucket's UUID-based webhook IDs
• Adds Bitbucket credential form (username + app password) to the frontend, and includes Bitbucket in add/import repository dialogs
• Removes "Experimental" badge for Bitbucket in Git Providers settings