feat: add partial unique index for single primary address per client

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

src/lib/db/schema/clients.ts

@@ -6,6 +6,7 @@ import {
   numeric,
   jsonb,
   index,
+  uniqueIndex,
   primaryKey,
 } from 'drizzle-orm/pg-core';
 import { sql } from 'drizzle-orm';
@@ -173,7 +174,13 @@ export const clientAddresses = pgTable(
     createdAt: timestamp('created_at', { withTimezone: true }).notNull().defaultNow(),
     updatedAt: timestamp('updated_at', { withTimezone: true }).notNull().defaultNow(),
   },
-  (table) => [index('idx_ca_client').on(table.clientId), index('idx_ca_port').on(table.portId)],
+  (table) => [
+    index('idx_ca_client').on(table.clientId),
+    index('idx_ca_port').on(table.portId),
+    uniqueIndex('idx_ca_primary')
+      .on(table.clientId)
+      .where(sql`${table.isPrimary} = true`),
+  ],
 );
 
 export type Client = typeof clients.$inferSelect;