Fix migration head conflict causing 503 errors

[bokelley]

Critical Bug Fix

Issue

The progress tracking feature (PR #508) created a migration head conflict causing 503 errors in production.

Symptoms

• ❌ 503 errors when polling sync status
• ❌ Migration error: "Multiple head revisions are present for given argument 'head'"
• ❌ Database migrations failing on deployment
• ❌ progress column never created in production database
• ❌ Code accessing sync_job.progress causing AttributeError → 503

Root Cause

Two migrations both pointed to the same parent (1a7693edad5d), creating a branch:

1a7693edad5d (branchpoint)
├─→ 661c474053fa (add_gam_service_account_email)
└─→ ed7d05fea3be (add progress tracking) ← CONFLICT!

Alembic couldn't determine which migration to run when executing alembic upgrade head.

Fix

Updated progress tracking migration to depend on 661c474053fa instead of 1a7693edad5d, creating a linear chain:

1a7693edad5d → 661c474053fa → ed7d05fea3be ✅

Changes

File: alembic/versions/ed7d05fea3be_add_progress_tracking_to_sync_jobs.py

• Changed down_revision from "1a7693edad5d" to "661c474053fa"

Verification

$ uv run alembic heads
ed7d05fea3be (head)  # ✅ Single head!

Impact

• ✅ Migration will run successfully on next deploy
• ✅ progress column will be created in database
• ✅ 503 errors will be resolved
• ✅ Progress tracking will work correctly
• ✅ AccuWeather sync can complete and report progress

Testing

• Pre-commit hooks: ✅ Passed (including "Check for multiple Alembic migration heads")
• Alembic heads check: ✅ Single head confirmed
• Migration history: ✅ Linear chain verified

Priority

🚨 HIGH - Blocks progress tracking feature from working in production

Related

• PR Add real-time progress tracking to inventory sync #508: Add real-time progress tracking to inventory sync
• Current issue: AccuWeather sync running but can't report progress (503 errors)

🤖 Generated with Claude Code

[bokelley]

Additional Fix Added: 409 Status Handling

New commit: 8b083da8 - Fix 409 handling for sync resume functionality

Issue

When a sync was already running and user clicked "Sync Inventory" again:

• Backend correctly returned 409 (Conflict) with {in_progress: true, sync_id: "..."}
• But fetch() treated 409 as error by default
• Response went to .catch() block instead of being parsed
• User saw console error: "Failed to load resource: 409"
• Seamless resume didn't work

Fix

Modified fetch response handler to accept 409 as valid:

.then(response => {
    // Handle both success and 409 (conflict) responses
    if (response.ok || response.status === 409) {
        return response.json();
    }
    throw new Error(`Server error: ${response.status}`);
})

Result

Now when user returns to page and clicks sync button:

1. ✅ Backend returns 409 with existing sync_id
2. ✅ Frontend parses JSON (no error)
3. ✅ Detects data.in_progress === true
4. ✅ Automatically resumes polling
5. ✅ Shows current progress: "⏳ Discovering Ad Units: 243 items (1/6)"
6. ✅ No console errors, no alerts

User experience: Seamless resume - feels like progress just continues!

[bokelley]

This PR has been merged and deployed. The 409 handling fix has been split into a separate PR #512 for cleaner review.