Custom CSV columns with values like "true"/"false" were stored as
strings, which made `ContactService.getAvailableFields()` report them
as string fields and lost the boolean toggle in the dashboard's
segment-filter UI. Only the reserved `subscribed` column was coerced.
Mirror the truthy keyword set already used for `subscribed`
(`true`/`1`/`yes`) and add its symmetric falsy counterpart
(`false`/`0`/`no`) for custom columns. Values outside that keyword set
are returned unchanged so names, IDs, or arbitrary strings are not
corrupted into `false`. Coercion runs over `Object.entries(customData)`
right before the `upsert()` call; no other code path needs to change
because `ContactService.upsert()` already accepts
`Record<string, unknown>` and `mergeContactData()` accepts mixed JSON
primitives. Post-import inference via `jsonb_typeof()` then classifies
the stored JSON boolean as type `boolean` automatically.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
