Refactor/intents repository interface

[Keengfk]

Summary

Introduces a IIntentsRepository interface and an InMemoryIntentsRepository
adapter, decoupling persistence from orchestration logic in IntentsService.
This is the prerequisite for the database work (#36) and on-chain state
reconciliation (#9) to land independently — each can provide its own adapter
bound to the INTENTS_REPOSITORY token without either touching IntentsService.

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Changes

src/intents/intents.repository.ts (new)
Defines the IIntentsRepository interface (6 methods: save, findById, findAll,
findByState, findByUser, update) and the INTENTS_REPOSITORY Symbol injection
token used to bind and inject the adapter.

src/intents/in-memory-intents.repository.ts (new)
The existing Map-based storage and seed logic lifted directly from
IntentsService into an @Injectable() class that implements IIntentsRepository.
Behaviour is identical — this is a pure structural move.

src/intents/intents.service.ts (modified)
Stripped to pure orchestration: UUID generation, state: "open" default, and
deadline fallback live here. All this.intents.* calls replaced with this.repo.*
via the injected IIntentsRepository. No logic was added or removed.

src/intents/intents.module.ts (modified)
Registers { provide: INTENTS_REPOSITORY, useClass: InMemoryIntentsRepository }.
To swap to a different backend (Prisma, on-chain), only this single binding
needs to change — nothing above it is touched.

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How to add a new storage adapter

// e.g. prisma-intents.repository.ts
@Injectable()
export class PrismaIntentsRepository implements IIntentsRepository {
constructor(private readonly prisma: PrismaService) {}
// ... implement the 6 methods
}

// intents.module.ts — one line change:
{ provide: INTENTS_REPOSITORY, useClass: PrismaIntentsRepository }

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Testing

• src/intents/in-memory-intents.repository.spec.ts (new) — 14 unit tests
  covering every method: seed count, save/overwrite, findById miss, sort order,
  state filter, case-insensitive user lookup, partial patch, update-miss
• src/intents/intents.service.spec.ts (updated) — now wires IntentsService via
  Test.createTestingModule with InMemoryIntentsRepository injected under the
  token, matching production DI exactly; added deadline default test

npm run lint ✓ 0 errors
npm run typecheck ✓ 0 errors
npm run build ✓ clean
npm test ✓ 35 tests, 5 suites (+13 vs baseline)
npm run test:e2e ✓ 23 tests, 5 suites

closes #56

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