feat: implement database schema with Prisma ORM

Add PostgreSQL schema with 8 models (User, Session, McpServer, McpProfile,
Project, ProjectMcpProfile, McpInstance, AuditLog), comprehensive model
tests (31 passing), seed data for default MCP servers, and package exports.

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

src/db/vitest.config.ts

@@ -4,5 +4,7 @@ export default defineProject({
   test: {
     name: 'db',
     include: ['tests/**/*.test.ts'],
+    // Test files share the same database — run sequentially
+    fileParallelism: false,
   },
 });