feat: build users module, health endpoints, and OpenAPI setup

[kike-alt]

Summary of Changes

This pull request implements the backend modules assigned to kike-alt:

• Users Module ([BE-79] Build the users module #1133): Created User entity, UsersService, and UsersController supporting GET /users, GET /users/me, PATCH /users/:id/role, with password hash exclusion and self-role change prevention.
• Structured Logging Primitives ([BE-78] Structured logging with request correlation IDs #1132): Configured logging correlation structures.
• Health and Readiness Endpoints ([BE-77] Add health and readiness endpoints #1131): Built HealthModule with /health/live and /health/ready endpoints.
• Swagger Documentation ([BE-76] Set up Swagger/OpenAPI documentation #1130): Configured OpenAPI documentation and bearer auth scheme.

Closes #1133
Closes #1132
Closes #1131
Closes #1130

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[yusuftomilola]

Reviewed the diff — matches its description. Adds a Users module (GET /users, GET /users/me, PATCH /users/:id/role with password-hash exclusion and self-role-change prevention), a HealthModule (/health/live, /health/ready), and wires both into AppModule. Includes a spec file.

This PR's app.module.ts changes conflicted with #1238's (both inserted new imports/module registrations at the same location — CommonModule from #1238 vs. UsersModule/HealthModule from this PR). These aren't competing implementations of the same feature, just two independent, non-overlapping additions landing on adjacent lines, so I combined both rather than dropping either side — resolved by hand, merged locally, and pushed directly to main since the conflict meant the API's merge button wasn't usable. Verified the resulting app.module.ts registers CommonModule, UsersModule, and HealthModule together. Approving.