feat: add flexible billing frequency support for invoices

Assessment groups can now define billing frequency (monthly, quarterly,
annual) with configurable due months and due day. Invoice generation
respects each group's schedule - only generating invoices when the
selected month is a billing month for that group. Adds a generation
preview showing which groups will be billed, period tracking on
invoices, and billing period context in the payments UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db/migrations/011-invoice-billing-frequency.sql

@@ -0,0 +1,57 @@
+-- Migration 011: Add billing frequency support to invoices
+-- Adds due_months and due_day to assessment_groups
+-- Adds period_start, period_end, assessment_group_id to invoices
+
+DO $$
+DECLARE
+  v_schema TEXT;
+BEGIN
+  FOR v_schema IN
+    SELECT schema_name FROM information_schema.schemata
+    WHERE schema_name LIKE 'tenant_%'
+  LOOP
+    -- Add due_months and due_day to assessment_groups
+    EXECUTE format('
+      ALTER TABLE %I.assessment_groups
+        ADD COLUMN IF NOT EXISTS due_months INTEGER[] DEFAULT ''{1,2,3,4,5,6,7,8,9,10,11,12}'',
+        ADD COLUMN IF NOT EXISTS due_day INTEGER DEFAULT 1
+    ', v_schema);
+
+    -- Add period tracking and assessment group link to invoices
+    EXECUTE format('
+      ALTER TABLE %I.invoices
+        ADD COLUMN IF NOT EXISTS period_start DATE,
+        ADD COLUMN IF NOT EXISTS period_end DATE,
+        ADD COLUMN IF NOT EXISTS assessment_group_id UUID
+    ', v_schema);
+
+    -- Backfill due_months based on existing frequency values
+    EXECUTE format('
+      UPDATE %I.assessment_groups
+      SET due_months = CASE frequency
+        WHEN ''quarterly'' THEN ''{1,4,7,10}''::INTEGER[]
+        WHEN ''annual'' THEN ''{1}''::INTEGER[]
+        ELSE ''{1,2,3,4,5,6,7,8,9,10,11,12}''::INTEGER[]
+      END
+      WHERE due_months IS NULL OR due_months = ''{1,2,3,4,5,6,7,8,9,10,11,12}''
+        AND frequency != ''monthly''
+    ', v_schema);
+
+    -- Backfill period_start/period_end for existing invoices (all monthly)
+    EXECUTE format('
+      UPDATE %I.invoices
+      SET period_start = invoice_date,
+          period_end = (invoice_date + INTERVAL ''1 month'' - INTERVAL ''1 day'')::DATE
+      WHERE period_start IS NULL AND invoice_type = ''regular_assessment''
+    ', v_schema);
+
+    -- Backfill assessment_group_id on existing invoices from units
+    EXECUTE format('
+      UPDATE %I.invoices i
+      SET assessment_group_id = u.assessment_group_id
+      FROM %I.units u
+      WHERE i.unit_id = u.id AND i.assessment_group_id IS NULL
+    ', v_schema, v_schema);
+
+  END LOOP;
+END $$;