Add proactive financial intelligence feed

Shifts the AI assistant from reactive (users ask questions) to proactive
(the system surfaces personalized insights automatically). A nightly job
analyzes every family's financial data across 7 insight types, writes
natural-language explanations via Claude, and surfaces them in a feed on
the dashboard and a standalone /insights page.

Feature is behind a flag: off by default, enable with INSIGHTS_ENABLED=1
(or via Setting.insights_enabled in the admin UI).

Insight types:
- spending_anomaly: category spend >25% above/below 3-month rolling average
- cash_flow_warning: projected cash balance drops below $500 in 30 days
  (uses RecurringTransaction + statistical daily baseline)
- net_worth_milestone: crossed a round-number milestone or all-time high
- subscription_audit: recurring transaction overdue by 45+ days
- savings_rate_change: savings rate changed >5 percentage points vs last month
- idle_cash: $5k+ sitting in depository account with no activity in 60 days
- budget_at_risk / budget_on_track: spending pace vs monthly budget

Architecture:
- Insight model with dedup_key unique index (upsert, not re-create daily)
- Insight::Generator base class + Insight::GeneratorRegistry orchestrator
- LLM used as a writer only — financial math runs in pure Ruby
- GenerateInsightsJob runs at 6 AM UTC daily via sidekiq-cron
- InsightsController with read/dismiss Turbo Stream actions
- Dashboard section gated by Current.user.insights_enabled?

https://claude.ai/code/session_014vY9xohpm3abSAxVxRF27a

app/models/setting.rb

@@ -129,6 +129,9 @@ class Setting < RailsSettings::Base
   field :auto_sync_time, type: :string, default: ENV.fetch("AUTO_SYNC_TIME", "02:22")
   field :auto_sync_timezone, type: :string, default: ENV.fetch("AUTO_SYNC_TIMEZONE", "UTC")
 
+  # Proactive financial insights feed (opt-in via INSIGHTS_ENABLED=1 env var or admin toggle)
+  field :insights_enabled, type: :boolean, default: ENV.fetch("INSIGHTS_ENABLED", "0") == "1"
+
   AUTO_SYNC_TIME_FORMAT = /\A([01]?\d|2[0-3]):([0-5]\d)\z/
 
   def self.valid_auto_sync_time?(time_str)