feat(modules): v1 feature-module gating + Investments

Exploration spike following expert-panel synthesis. Codifies the 3-tier
gating model (instance / per-user / per-family) without adding a
framework. Investments is the first family-scoped module.

- Family#disabled_modules: string[] column, opt-out semantics. No
  per-module DB column. Existing families default to [] = enabled.
- Family::AVAILABLE_MODULES = %w[investments] (the registry).
- ModuleGateable concern (auto-included in ApplicationController):
  require_module! class macro + module_enabled? helper.
- Api::V1::BaseController#require_module!: 403 feature_disabled JSON,
  mirrors require_ai_enabled.
- NavigationHelper extracts mobile/desktop nav into a single source
  with module: key support; mobile shrinks via justify-around, never
  auto-fills empty slots.
- Settings → Preferences gains a family-scoped module toggle card.
- 4 HTML controllers (Investments, Holdings, Trades, Securities) +
  3 API controllers gated. Investment/Crypto account types hidden in
  the new-account modal when off.
- docs/feature-gating.md codifies the rule for future modules.

Background-job layer not wired (no Investments-specific scheduled job
to gate; flagged as TODO in docs). Run db:migrate before bin/rails
test. No PR yet — awaiting decisions in open-questions list.

app/controllers/api/v1/securities_controller.rb

@@ -4,6 +4,7 @@ class Api::V1::SecuritiesController < Api::V1::BaseController
   include Pagy::Backend
   include Api::V1::SecurityResourceFiltering
 
+  before_action -> { require_module!(:investments) }
   before_action :ensure_read_scope
   before_action :set_security, only: :show