feat: expand DI container with lifecycle management and domain modules

[jajafwangshak86-ops]

Closes #485

What's changed

backend/src/di/container.ts

• Lifecycle management: singleton (shared), transient (new per call), scoped (shared within a request scope)
• DIContainer.createFresh() — isolated instance for test mock injection, no shared state
• container.validate() — iterates all registrations at startup; throws on misconfigured factories
• container.reset() — clears singleton cache (tests)
• Backwards-compatible: existing get(), set(), has(), and typed getters unchanged

backend/src/di/modules/

Per-domain module files that call c.register():

• project.module.ts — ProjectRepository → ProjectService → ProjectController
• auth.module.ts — LockoutManager, AuditService
• payment.module.ts — AnalyticsService, VerificationService, InvoiceService

backend/src/di/index.ts — bootstrapDI()

• Loads all domain modules
• Runs validate() on startup (opt-out via DI_VALIDATION=false env var)

backend/src/di/__tests__/container.test.ts

• All three lifecycles
• Mock injection override
• Transitive dependency graph resolution
• Validation pass/fail
• Reset (singleton cache cleared)
• Performance: singleton resolve < 1ms

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