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>

backend/src/database/tenant-schema.service.ts

@@ -112,6 +112,8 @@ export class TenantSchemaService {
         special_assessment DECIMAL(10,2) DEFAULT 0.00,
         unit_count INTEGER DEFAULT 0,
         frequency VARCHAR(20) DEFAULT 'monthly' CHECK (frequency IN ('monthly', 'quarterly', 'annual')),
+        due_months INTEGER[] DEFAULT '{1,2,3,4,5,6,7,8,9,10,11,12}',
+        due_day INTEGER DEFAULT 1,
         is_default BOOLEAN DEFAULT FALSE,
         is_active BOOLEAN DEFAULT TRUE,
         created_at TIMESTAMPTZ DEFAULT NOW(),
@@ -157,6 +159,9 @@ export class TenantSchemaService {
         status VARCHAR(20) DEFAULT 'draft' CHECK (status IN (
           'draft', 'sent', 'paid', 'partial', 'overdue', 'void', 'written_off'
         )),
+        period_start DATE,
+        period_end DATE,
+        assessment_group_id UUID REFERENCES "${s}".assessment_groups(id),
         journal_entry_id UUID REFERENCES "${s}".journal_entries(id),
         sent_at TIMESTAMPTZ,
         paid_at TIMESTAMPTZ,