What does this PR do?

Adds support for site-specific byline metadata — Twitter handle, pronouns,
company, localised job title, etc. — via a new /byline-schema admin
screen. Sites can register custom fields with their own type, validation,
and per-locale storage policy without editing code; values surface on
BylineSummary.customFields for the frontend.

Custom-field values can be set at both create and update time through
the same customFields map on POST and PUT /_emdash/api/admin/bylines.
In the admin, registered fields render inline with Name, Bio, etc. — no
"Custom fields" section header — and are available in both the New byline
and Edit byline dialogs. The schema link sits at the top of the
Bylines page (admin-only) rather than the global sidebar so admins find
it in context.

Design choices worth flagging

• Schema in the database, not config. _emdash_byline_fields is the
  source of truth, with options.byline_fields_version driving cache
  invalidation. Admins register fields through the admin UI; no code
  edits required.
• Per-field translatable flag. Translatable values are stored per
  locale (one row per locale in the byline's translation_group).
  Non-translatable values are stored once per translation_group and
  surface on every locale variant. The flag is locked once values
  reference the field — flipping would orphan rows in the wrong table.
• url scheme allowlist. URL fields require http: or https:;
  javascript:/data:/mailto:/ftp:/file: reject. Mirrors
  httpUrl in api/schemas/common.ts — custom URLs typically render
  as <a href> so this closes the XSS footgun.
• Create + update share one transaction on Node/PG. Byline row
  write and per-field writes roll back together on partial failure.
• D1 retry-safe create via idempotent recovery. D1 has no
  transactions, so a crash between the row insert and the field writes
  leaves a partial byline that the API would otherwise refuse to
  recover from. A retry POST is treated as completing the abandoned
  create iff the full fixed-column payload, the translation-group
  identity, and the existing custom-field subset all match the
  incoming request. Anything else collapses to a standard
  duplicate-slug CONFLICT — recovery only fires when the retry is
  provably the same request as the original.
• Cache coherency via parity bit. options.byline_fields_version
  carries meaning in its parity — odd = mutation in flight (or
  crashed), even = stable. The cache bypasses the global holder while
  odd. markVersionDirty is parity-aware idempotent; markVersionClean
  always advances to a new even value so two concurrent mutators
  can't collapse on the same key and pin a stale snapshot.
  Idempotent-retry exits also run markVersionClean — same code path
  doubles as crash recovery and false-clean recovery.

BylineSummary gains an optional
customFields: Record<string, CustomFieldValue>. Existing
object-literal consumers stay source-compatible — the property is
optional and runtime always returns {} when no fields are registered.
Builds on the bylines-i18n foundation from #1146.

What's deferred

• required flag enforcement. The admin UI exposes a Required
  toggle and the value persists on the field definition, but the
  server accepts missing values and null for any field. Needs a
  design call on the enforcement model (reject on first set only?
  reject on every save? coerce vs reject?). TODOs are in
  BylineRepository.coerceFieldValue and BylineFieldEditor.
• Full multi-isolate concurrency on schema mutations. The
  parity-aware bookend has a residual race between two concurrent
  markVersionClean calls — bounded by the inter-clean duration
  (~ms). Schema mutations are admin-only and rare; acceptable for
  this PR. A CAS-on-bump or dialect-specific row lock would close
  it but raises dialect-divergence scope.
• Idempotency keys for D1 retries. The recovery branch carries
  multiple conjunctive guards approximating "same request as before"
  via state inspection. If similar D1 retry-safety problems recur on
  other create endpoints, an explicit Idempotency-Key header would
  replace the apparatus with one lookup. Worth a Discussion before
  the next D1 retry surface comes up.
• translationOf-flavoured create recovery. The sibling-locale
  guard returns CONFLICT before the recovery branch can fire on
  translation creates, so partial translationOf retries fall through
  to manual recovery via the edit flow. Uncommon enough that
  reordering the guard wasn't justified for this PR.

Closes #1174

Closes #

Type of change

• Bug fix
• Feature (requires maintainer-approved Discussion)
• Refactor (no behavior change)
• Translation
• Documentation
• Performance improvement
• Tests
• Chore (dependencies, CI, tooling)

Checklist

• I have read CONTRIBUTING.md
• pnpm typecheck passes
• pnpm lint passes
• pnpm test passes (or targeted tests for my change)
• pnpm format has been run
• I have added/updated tests for my changes (if applicable)
• User-visible strings in the admin UI are wrapped for translation (if applicable). Do not include messages.po changes except in translation PRs — a workflow extracts catalogs on merge to main.
• I have added a changeset (if this PR changes a published package)
• New features link to an approved Discussion: Custom fields on bylines #1174

AI-generated code disclosure

• This PR includes AI-generated code — model/tool: Claude Opus 4.7, GPT-5.5

Screenshots / test output