feat: consolidate prisma client usage

[Lansa-18]

Closes #116

Consolidate Prisma Client to Shared Singleton (#116)

Summary

Consolidates all Prisma client usage to the shared singleton in src/db/index.ts. Previously, 6 modules each called new PrismaClient() directly, creating extra connection pools and making graceful shutdown fragile.

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Changes Implemented

Converted to Singleton — import db from '../db'

• src/stellar/events.ts
• src/agent/snapshotter.ts
• src/agent/router.ts
• src/agent/loop.ts
• src/agent/scanner.ts
• src/jobs/sessionCleanup.ts

Graceful Shutdown Fixed — src/agent/loop.ts

• Removed await prisma.$disconnect() from stopAgentLoop() — the singleton's disconnect is now handled exclusively by src/index.ts after stopAgentLoop() returns, eliminating duplicate disconnects
• Removed the duplicate SIGTERM/SIGINT signal handlers from setupGracefulShutdown() — these were racing with src/index.ts's own gracefulShutdown() and calling process.exit(0) prematurely. Only the uncaughtException handler is kept

Test Mocks Updated

File	Change
src/stellar/__tests__/events.helpers.test.ts	Replaced the now-unused PrismaClient mock with jest.mock('../../db', () => ({ default: {} }))
tests/unit/stellar/events.test.ts	Added jest.mock('../../../src/db', ...) pointing to the existing mockPrisma instance

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Technical Decisions

• Pure infrastructure refactor — no business logic changes. Query behaviour is identical since the singleton uses the same @prisma/client under the hood
• After this change, a single db.$disconnect() in src/index.ts cleanly closes the one shared pool on SIGTERM/SIGINT
• Shutdown sequence is now unambiguously:

  stop event listener → stop agent loop (cron jobs only) → disconnect Prisma

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