Introduce `packages/shared` for front/back contracts

[arediss]

Context

Currently packages/ contains backend/, frontend/, plugins/ — no shared package.

Concrete duplication example:

• ACTIVE_REQUEST_STATUSES defined identically (['pending', 'approved', 'processing', 'failed']) in:
  • packages/frontend/src/utils/requestStatus.ts:2
  • packages/backend/src/utils/requestStatus.ts:13

Risk: silent divergence at the next workflow change.

Scope

• Full audit of constants/types duplicated front↔back (statuses, enums, DTOs)
• Create packages/shared (ESM, strict TS, no heavy deps)
• Migrate progressively: ACTIVE_REQUEST_STATUSES first, then cross-cutting DTO types
• Update workspaces / tsconfig

Success criteria

• Single source of truth for cross-cutting contracts
• 0 back↔front duplication on shareable constants

Priority

Medium — launch after the audit if at least 3-4 real duplications are identified.

Parent: #145