feat(scanning): JIRA + Linear ticket creation per finding

[Metbcy]

Problem

Triage workflow (v0.7.0) ships in-app status + comments per finding. For teams whose source-of-truth is JIRA or Linear, an extra step is required to manually create tickets. SecureScan should be able to file a ticket from the triage panel.

Acceptance criteria

• New /settings/integrations page (sibling to webhooks/keys) for JIRA + Linear API tokens.
• 'Create JIRA ticket' / 'Create Linear ticket' button on each finding's expanded view.
• One-time token storage matches the API-key one-shot reveal pattern.
• Created tickets get linked back to the finding via the existing metadata field; finding's expanded view shows the ticket link.
• If both integrations are configured, user picks one per click.

Suggested approach

JIRA: POST /rest/api/3/issue (Atlassian API), token via API token + email.
Linear: GraphQL mutation issueCreate, token via personal access token.

Both have well-documented Python clients; thin wrapper modules in backend/securescan/integrations/{jira,linear}.py.

Difficulty

~1 day per integration.