feat: accounting module — schema, migration, Zod schemas, AR balance service

Phase 1 foundation:
- Migration 0047: all accounting tables (invoice, payment_application,
  account_balance, account_code, journal_entry, billing_run), chart of
  accounts seed, nextBillingDate on enrollment, accounting module config
- Drizzle schema file with all table definitions and type exports
- Zod validation schemas (invoice, payment, billing, reports)
- AccountBalanceService: materialized AR balance per account

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/backend/src/services/account-balance.service.ts

@@ -0,0 +1,93 @@
+import { eq, desc, gt, count } from 'drizzle-orm'
+import type { PostgresJsDatabase } from 'drizzle-orm/postgres-js'
+import { accountBalances } from '../db/schema/accounting.js'
+import { accounts } from '../db/schema/accounts.js'
+import type { PaginationInput } from '@lunarfront/shared/schemas'
+
+export const AccountBalanceService = {
+  async getBalance(db: PostgresJsDatabase<any>, accountId: string) {
+    const [existing] = await db.select().from(accountBalances).where(eq(accountBalances.accountId, accountId)).limit(1)
+    if (existing) return existing
+
+    // Create balance record if it doesn't exist
+    const [created] = await db.insert(accountBalances).values({ accountId }).returning()
+    return created
+  },
+
+  async adjustBalance(
+    db: PostgresJsDatabase<any>,
+    accountId: string,
+    adjustment: number,
+    reason: 'invoice' | 'payment' | 'void' | 'write_off',
+  ) {
+    const balance = await this.getBalance(db, accountId)
+    const newBalance = parseFloat(balance.currentBalance) + adjustment
+
+    const updates: Record<string, unknown> = {
+      currentBalance: newBalance.toFixed(2),
+      updatedAt: new Date(),
+    }
+
+    if (reason === 'invoice' && adjustment > 0) {
+      updates.lastInvoiceDate = new Date().toISOString().slice(0, 10)
+    }
+    if (reason === 'payment' && adjustment < 0) {
+      updates.lastPaymentDate = new Date().toISOString().slice(0, 10)
+    }
+
+    await db.update(accountBalances).set(updates).where(eq(accountBalances.accountId, accountId))
+  },
+
+  async getOutstandingAccounts(db: PostgresJsDatabase<any>, params: PaginationInput) {
+    const where = gt(accountBalances.currentBalance, '0')
+    const offset = ((params.page ?? 1) - 1) * (params.limit ?? 25)
+
+    const [data, [{ total }]] = await Promise.all([
+      db.select({
+        accountId: accountBalances.accountId,
+        currentBalance: accountBalances.currentBalance,
+        lastInvoiceDate: accountBalances.lastInvoiceDate,
+        lastPaymentDate: accountBalances.lastPaymentDate,
+        accountName: accounts.name,
+        accountEmail: accounts.email,
+      })
+        .from(accountBalances)
+        .innerJoin(accounts, eq(accountBalances.accountId, accounts.id))
+        .where(where)
+        .orderBy(desc(accountBalances.currentBalance))
+        .limit(params.limit ?? 25)
+        .offset(offset),
+      db.select({ total: count() })
+        .from(accountBalances)
+        .where(where),
+    ])
+
+    return {
+      data,
+      pagination: {
+        page: params.page ?? 1,
+        limit: params.limit ?? 25,
+        total,
+        totalPages: Math.ceil(total / (params.limit ?? 25)),
+      },
+    }
+  },
+
+  async recalculateFromInvoices(db: PostgresJsDatabase<any>, accountId: string) {
+    // Safety valve: recalculate balance from all invoices
+    const { invoices } = await import('../db/schema/accounting.js')
+    const rows = await db
+      .select({ balance: invoices.balance, status: invoices.status })
+      .from(invoices)
+      .where(eq(invoices.accountId, accountId))
+
+    const totalOutstanding = rows
+      .filter(r => ['sent', 'partial', 'overdue'].includes(r.status))
+      .reduce((sum, r) => sum + parseFloat(r.balance), 0)
+
+    await db.update(accountBalances).set({
+      currentBalance: totalOutstanding.toFixed(2),
+      updatedAt: new Date(),
+    }).where(eq(accountBalances.accountId, accountId))
+  },
+}