[driaug]

Description

Closes #487. Replaces #490, whose analysis and single-pass query this builds on.

GET /contacts/fields scanned every contact in the project on each request and re-read the table once per custom field, so the segment builder, workflow condition editor and template editor were slow to open on large projects.

• Field names are registered on write. ContactService.create, update, upsert and the workflow "update contact" step add the Contact.data keys they store to a per-project sorted set in Redis (one Lua call per write). A new field appears in the pickers immediately.
• Coverage and type are measured in the background. A new contact-fields BullMQ job runs one pass over the project's contacts and stores the result. When it finishes, it also removes keys no contact carries any more. A key first written while a scan is running is kept.
• The endpoint only reads Redis. It queues a scan when the measurements are missing, older than an hour, or lack a field.
• It throttles itself, with no configuration. Scans triggered by writes are debounced by 10s, so an import collapses into one scan. A project is scanned at most once every 5 minutes. Idle projects expire from Redis after 30 days.
• Deleting a field removes it from the list immediately.
• The segment builder loads its options through SWR, so reopening it no longer refetches everything.

Behaviour changes

• A field that has not been measured yet reports coverage: null. The editor menus and template lint skip it, and the data settings table shows "—".
• Until a project's first scan completes, the response has pending: true and lists only the standard fields plus fields written since. The frontend polls every 3s while pending.
• A stored JSON null no longer counts toward coverage, and a field holding mixed types reports as string.

Type of Change

• feat: New feature (MINOR version bump)
• fix: Bug fix (PATCH version bump)
• feat!: Breaking change - new feature (MAJOR version bump)
• fix!: Breaking change - bug fix (MAJOR version bump)
• docs: Documentation update (no version bump)
• chore: Maintenance/dependencies (no version bump)
• refactor: Code refactoring (no version bump)
• test: Adding tests (no version bump)
• perf: Performance improvement (PATCH version bump)

Testing

• New ContactFieldService.test.ts (9 tests): coverage and type, a new field offered before it is measured, no scan for known fields, removal of unused keys, a key written during a scan, field deletion, project isolation.
• Workflows.test.ts now creates its contact through ContactService, since custom fields are registered by writes. Three test files that mock Redis got a defineCommand stub.
• Checked end to end against real BullMQ with a throwaway test: the worker ran the first scan, then pushed a follow-up scan back to the 5-minute mark.
• Full suite: 1652 passing. tsc --noEmit and ESLint clean for api and web.

Checklist

• PR title follows conventional commits format
• Code builds successfully
• Tests pass locally
• Documentation updated (if needed)

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