Implement OpenAPI specification for the GLAPAGOS platform API

[castroquiles]

Context

The platform API lives under `src/api/`. Currently the directory is stubbed
with no specification. Before any implementation work begins, the API contract
needs to be defined in a machine-readable format that downstream consumers,
SDK authors, and governance tooling can depend on.

Objective

Produce a complete OpenAPI 3.1 specification for the GLAPAGOS platform API
v0.1, covering the domains documented in the platform architecture.

Scope

The spec should cover the following resource groups at minimum:

• `/initiatives` -- regional AI governance initiatives (read operations)
• `/benchmarks` -- platform evaluation benchmarks (read operations)
• `/working-groups` -- active working groups and membership (read operations)
• `/chapters` -- regional chapters and their status (read operations)

Each endpoint must include:

• Request and response schemas using JSON Schema (draft 2020-12)
• Documented error responses (400, 404, 422, 500)
• Examples for at least one success and one error response per endpoint
• Security scheme definition (Bearer token, even if auth is not yet implemented)

Deliverable

A single file at `src/api/openapi.yaml` that passes validation against the
OpenAPI 3.1 schema without warnings.

Validation

```bash
npx @redocly/cli lint src/api/openapi.yaml
```

References

• Platform domains: `README.md` Core Domains section
• Architecture: `docs/architecture/`
• OpenAPI 3.1 spec: https://spec.openapis.org/oas/v3.1.0

Notes

This issue does not require backend implementation. The spec is the
deliverable. Implementation follows in a separate issue once the spec
is reviewed and merged.

A passing PR will have:

• `src/api/openapi.yaml` added
• `src/api/README.md` updated to reference the spec
• No changes outside `src/api/`