fix: close release preflight gaps

Route public health checks to the API, backfill and synchronize job opportunities, stabilize SPA smoke tests, and document operator-only production steps.

docs/operations/job-opportunity-cutover.md

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+# Job opportunity cutover
+
+The release-readiness build closes the write-side gap:
+
+- normal application creation and Gmail suggested-job creation both create a linked `Job`;
+- edits synchronize opportunity fields through `JobOpportunitySync`;
+- startup idempotently creates one `Job` for every legacy `JobApplication` whose `JobId` is null.
+
+The startup backfill is additive and runs after the deployment backup. It does not drop or overwrite
+legacy application columns.
+
+## Production validation
+
+Run these read-only checks after deployment. Expected result for every count is `0`:
+
+```sql
+SELECT COUNT(*) AS missing_job
+FROM JobApplications
+WHERE JobId IS NULL;
+
+SELECT COUNT(*) AS missing_target
+FROM JobApplications a
+LEFT JOIN Jobs j ON j.Id = a.JobId
+WHERE a.JobId IS NOT NULL AND j.Id IS NULL;
+
+SELECT COUNT(*) AS owner_mismatch
+FROM JobApplications a
+JOIN Jobs j ON j.Id = a.JobId
+WHERE NOT (a.OwnerUserId <=> j.OwnerUserId);
+
+SELECT COUNT(*) AS field_mismatch
+FROM JobApplications a
+JOIN Jobs j ON j.Id = a.JobId
+WHERE NOT (a.CompanyId <=> j.CompanyId)
+   OR NOT (a.JobTitle <=> j.JobTitle)
+   OR NOT (a.Location <=> j.Location)
+   OR NOT (a.JobUrl <=> j.JobUrl)
+   OR NOT (a.Description <=> j.Description)
+   OR NOT (a.Salary <=> j.Salary);
+```
+
+`<=>` is MariaDB/MySQL's null-safe equality operator. Do not use these statements as an update script.
+
+## Expand/contract release order
+
+1. Deploy the additive backfill and synchronized writers.
+2. Capture the validation counts above and observe one release.
+3. Change reads to use `Job`; keep compatibility columns during that release.
+4. Re-run validation against a fresh backup restore.
+5. Only then make `JobId` required and drop duplicated opportunity columns in a later migration.
+
+Do not combine the destructive column drop with the first production backfill.