feat(settings): add organization settings and loyalty setting history

Migration 000091 creates organization_settings, a key-value store per
organization shaped like outlet_settings, for the loyalty settings that must
be the same in every outlet (point value, exchange rate, transfer limits,
expiry). Until now there was nowhere to keep organization-level settings.

Also creates loyalty_setting_changes, the append-only log of who changed
which loyalty setting from what to what (PRD F2), for both organization and
outlet settings (PC-102).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

migrations/000091_create_organization_settings.down.sql

@@ -0,0 +1,2 @@
+DROP TABLE IF EXISTS loyalty_setting_changes;
+DROP TABLE IF EXISTS organization_settings;

migrations/000091_create_organization_settings.up.sql

@@ -0,0 +1,35 @@
+-- Settings that must be the same in every outlet of an organization, starting with the
+-- loyalty ones (docs/prd-point-coin.md F2, F12): point value, exchange rate, transfer
+-- limits and expiry. Same key-value shape as outlet_settings.
+CREATE TABLE organization_settings (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    organization_id UUID NOT NULL REFERENCES organizations(id) ON DELETE CASCADE,
+    key VARCHAR(255) NOT NULL,
+    value TEXT,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    UNIQUE(organization_id, key)
+);
+
+-- The unique constraint leads with organization_id, so it also serves the plain
+-- per-organization lookups and there is no separate index on that column.
+
+-- Who changed which loyalty setting, from what, to what (F2). Covers both the
+-- organization settings above and the per-outlet loyalty keys in outlet_settings.
+-- Append-only. Values are stored as text, the same as in the settings tables.
+CREATE TABLE loyalty_setting_changes (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    organization_id UUID NOT NULL,
+    -- NULL for an organization setting.
+    outlet_id UUID,
+    key VARCHAR(100) NOT NULL,
+    -- NULL when the key had no stored value yet (it was on its default).
+    old_value TEXT,
+    new_value TEXT,
+    changed_by UUID NOT NULL,
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+CREATE INDEX idx_loyalty_setting_changes_organization_id_created_at ON loyalty_setting_changes(organization_id, created_at DESC);
+CREATE INDEX idx_loyalty_setting_changes_outlet_id_created_at ON loyalty_setting_changes(outlet_id, created_at DESC)
+    WHERE outlet_id IS NOT NULL;