Lays the foundation for Retirement v2 as a preview feature stacked on
Goals v2. Math, lens UI, pension sources and bucket all defer to later
PRs; this PR ships only the data-model spine and a placeholder landing.
- STI on goals: add `type` (default "Goal") + `user_id` columns;
partial index for `Goal::Retirement` rows; check constraint
requiring an owner on retirement rows. Existing goals backfill to
`type='Goal'`; base `Goal#editable_by?` stays family-scoped.
- `Goal::Retirement` subclass with single-user owner and
`editable_by?` narrowed to owner-only. Parent depository-only
linked-account validations no-op'd; PR2 introduces
`RetirementBucketEntry`.
- `families.retirement_disabled` killswitch (default false) +
`Family#retirement_enabled?(user)` helper as tier 2 of the gate.
Tier 1 is the existing `PreviewGateable` flow.
- `RetirementController#show`: `require_preview_features!` then
`ensure_module_enabled!` then a placeholder body. Unknown to users
without preview features; 404 when the family killswitch is on (the
feature behaves as if it does not exist).
- Sidebar: new `sun`-icon entry after Goals, hidden unless the user has
preview features AND the family has retirement enabled, so the
killswitch hides the nav rather than leaving a link that 404s.
- Locales: EN copy for nav, breadcrumb, page header, placeholder body,
and the new `owner.must_belong_to_family` validation message under
the goal model. DE deferred to PR4.
- Tests: STI roundtrip, owner presence + family-membership
validations, `editable_by?` on both Goal and Goal::Retirement, gate
matrix on the controller, nav-item visibility under both preview and
family flags, base-row STI backfill.
Stack ahead: PR2 ships the data plane (PensionSource, statements,
adjustments, bucket entries); PR3 wires the `Retirement::Fire::*`
forecast engine + WHAT-IF Turbo Stream slider loop; PR4 lands the
single combined-page UI per Claude's 2026-05-29 design (glide chart
with hover-tooltip income breakdown, no separate stacked-area chart).
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