feat: Add Linear hooks, action item endpoints, and backfill

[spalmurray]

PR 3 of 4 in the Linear action items series. Adds:

• Linear hooks: create parent issue on incident creation, sync title on title/visibility changes, redact private incident titles
• Action items list endpoint with sync-on-read
• Force sync endpoint (POST /incidents//sync-action-items/)
• Backfill: if an incident is missing a Linear connection when synced, creates one automatically
• DRYs up incident ID parsing across all views with parse_incident_id() helper
• Tests for hooks, views, backfill path

[cursor]

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