feat: Add Linear action item syncing for incidents

[spalmurray]

Adds backend support for tracking Linear action items on incidents. Users create issues in Linear and attach them to the Firetower incident URL. Firetower syncs these issues into a local ActionItem model with a throttled sync (same pattern as Slack participant sync). Details:

• New ActionItem model linked to incidents via Linear issue attachments
• LinearService that queries Linear's GraphQL API for issues attached to a given Firetower URL
• Throttled sync on the action items list endpoint, with a force sync endpoint
• Django admin action to sync action items (matches existing Slack participant sync pattern)
• Auto-creates users for Linear assignees who haven't logged into Firetower yet
• Extracts parse_incident_id helper to deduplicate incident ID parsing across views
• New firetower_base_url config to ensure attachment URL matching is specific enough to avoid cross-environment collisions (e.g. TESTINC-2000 vs INC-2000)

Before merge

• Add [linear] section with api_key and action_item_sync_throttle_seconds to production config
• Add [linear] section with empty api_key to config.ci.toml
• Ensure firetower_base_url is set correctly in production config (moved to top-level)
• Run migration 0015 (adds ActionItem model and action_items_last_synced_at to Incident)
• Obtain Linear workspace API key with attachment/issue read permissions
• Test sync in test env with a real Linear-attached incident

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[spalmurray]

this is blocked by Linear not having org level API keys. Am working on a different approach