GitLab integration: label linked issues and MRs

[emyller]

Context: #7000

When a GitLab issue or MR is linked to a feature flag (#7160), Flagsmith should add a "Flagsmith Flag" label to it so the team can filter for linked items in GitLab.

Scope

• Create label via POST /api/v4/projects/:id/labels. Handle 409 (label already exists on the project).
• Apply label via PUT /api/v4/projects/:id/issues/:iid with {"labels": "Flagsmith Flag"}.
• tagging_enabled on GitLabConfiguration as an integration-level toggle. Exposed in the frontend as a checkbox field — same pattern as Datadog's use_custom_source.

Important

In the GitHub integration, tagging_enabled controls two things: the "Flagsmith Flag" label on issues/PRs in GitHub, and feature state tags in Flagsmith (e.g. "PR Merged", "Issue Closed"). This issue covers the label side. The feature state tags are driven by webhooks and delivered in #7165. Both share the same tagging_enabled toggle.

Note

#7120 and #7121 contain reference code:

• feat(gitlab): add configuration persistence and resource browsing #7121 calls create_flagsmith_flag_label on config create/update, gated behind gitlab_project_id. That field no longer exists — the label should be created when a user links a resource, since that's when the project ID is known.
• feat(gitlab): add configuration persistence and resource browsing #7121 seeds Flagsmith-side feature tags (GitLabTag enum: MR Open, MR Merged, Issue Closed, etc.) via a lifecycle hook on tagging_enabled. This serves GitLab integration: receive webhooks for automatic state sync #7165 — do not include in this issue.

Acceptance criteria

• "Flagsmith Flag" label created on the GitLab project when first needed
• Label applied to issue or MR when linked to a flag
• Tagging can be toggled on/off from the integration settings