fix(users): secure GET /users and GET /users/:id against PII enumeration [Issue #64]

[gramseostudio-dev]

Closes #64

Description

This PR secures the GET /users and GET /users/:id endpoints to prevent PII enumeration, following a rigorous SDD (Specification-Driven Development) workflow.

Changes

• Security: Added JwtAuthGuard and @ApiBearerAuth() to GET /users and GET /users/:id to strictly require valid authentication.
• Data Protection: Created PublicUserDto and toPublicUser mapper to strip sensitive fields (such as email and stellarAddress) before reaching the HTTP response.
• Refactoring: Refactored UsersService to use the new toPublicUser mapper for public listings/lookups, while introducing findOneRaw to maintain backward compatibility with internal operations (like role assignments and github upserts).
• Testing & Infrastructure: Implemented and executed E2E integration tests in test/users.e2e-spec.ts, backed by a real PostgreSQL instance via Docker (docker compose up db -d), to assert that unauthenticated requests are properly rejected.

Verification Steps

• Database Setup: Spun up local PostgreSQL database via Docker (docker compose up db -d).
• E2E Testing: Run npm run test:e2e to verify that unauthenticated requests to user endpoints return unauthorized/forbidden responses as expected.
• Compilation: Run npm run build to confirm zero type or compilation errors across the NestJS backend graph.
• API Inspection: Manual validation ensuring sensitive fields like email are completely absent from public-facing responses.

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