fix: campaign_snapshots column name mismatch - campaign_p_id vs campaign_pid

[Jing-yilin]

Problem

The campaign_snapshots table has a column name mismatch causing snapshot storage to fail:

ERROR: column "campaign_pid" does not exist (SQLSTATE 42703)

Root cause:

• Model field: CampaignPID string
• GORM snake_case: campaign_p_id (incorrect due to PID → p_id)
• Expected: campaign_pid

This is the same issue we had with the campaigns table PK (#24), but we forgot to fix it for the foreign key in campaign_snapshots.

Impact

• ❌ All snapshot inserts fail (5,614 errors on each crawl)
• ❌ Sparkline charts have no historical data
• ❌ Velocity calculations broken

Current State

From logs (2026-02-27 14:12 UTC):

• ✅ 5,614 campaigns upserted successfully
• ❌ 5,614 snapshot inserts failed with campaign_pid error

Solution

Add idempotent migration in db.go:

1. Rename campaign_p_id → campaign_pid if exists
2. Update GORM tag: gorm:"column:campaign_pid"

Related

• feat(#18): add POST /api/admin/backfill for historical deep crawl #24 - Original campaigns.pid column name fix
• PR fix: GORM column name mismatch — Campaign.PID saves as p_id instead of pid #25 - Where we should have caught this