feat!: require project id and move client API under /projects/{projectID}

[jeroenrinzema]

Breaking change

The Lunogram Client API now requires the project UUID as a path segment.
Every authenticated endpoint moves from /api/client/... to
/api/client/projects/{projectID}/.... Auth is unchanged (same API key/token);
only the URL gains the project. This is a hard cut with no backwards
compatibility.

To match, the SDK now takes the project ID as a required construction
parameter. Callers supply it once when building the client; the SDK injects
/projects/{projectID} into every Client API path automatically. You never pass
the project per call.

Before / after

// Before
client := lunogram.NewClient("api-key")
user, err := client.UpsertUser(ctx, req)

// After — projectID (a UUID) is required; NewClient now returns an error
client, err := lunogram.NewClient("api-key", "11111111-1111-1111-1111-111111111111")
if err != nil {
    log.Fatal(err) // empty or non-UUID projectID
}
user, err := client.UpsertUser(ctx, req)

NewClient(apiKey, projectID string, opts ...Option) (*Client, error) validates
that projectID is a non-empty UUID and returns an error otherwise. Functional
options (WithBaseURL, WithHTTPClient, WithTimeout) are unchanged.

Path changes (old → new)

All Client API endpoints implemented by this SDK:

Old	New
/api/client/users	/api/client/projects/{projectID}/users
/api/client/users/events	/api/client/projects/{projectID}/users/events
/api/client/users/scheduled	/api/client/projects/{projectID}/users/scheduled
/api/client/organizations	/api/client/projects/{projectID}/organizations
/api/client/organizations/users	/api/client/projects/{projectID}/organizations/users
/api/client/organizations/events	/api/client/projects/{projectID}/organizations/events
/api/client/organizations/scheduled	/api/client/projects/{projectID}/organizations/scheduled

The prefix is injected in one place (Client.clientPath in client.go), so
resource methods now pass project-relative paths (e.g. /users). The broader
platform contract also moves inbox/devices/push/auth-methods endpoints under
/projects/{projectID}; those are not currently implemented in this SDK, so no
SDK code references them.

How it's wired

• Client gains a projectID field, set and validated in NewClient.
• clientPath(path) returns "/api/client/projects/" + projectID + path; the
  transport (do) builds the URL via c.baseURL + c.clientPath(path).
• Resource files (users.go, organizations.go, scheduled.go) pass
  project-relative paths — no per-call project handling.

Files touched

• client.go — required projectID, UUID validation, clientPath, transport.
• users.go, organizations.go, scheduled.go — project-relative paths.
• integration_test.go — construct via new signature; read/skip on LUNOGRAM_PROJECT_ID.
• README.md — project-scoping docs, updated quick start and option snippets.

Validation

go build ./..., go vet ./..., gofmt -l . (clean), and go test ./... all
pass. Integration tests skip without live-API env vars, as before.

Dependency

This depends on the corresponding platform change that requires the project in
the Client API URL. It should merge once that ships.

[jeroenrinzema]

Superseded by #3. The spec-driven SDK in #3 retargets to main and carries this project-in-URL facade (NewClient(apiKey, projectID), the /api/client/projects/{projectID} clientPath injection, and the UUID validation) in its history. Closing in favor of generating the client from the OpenAPI spec, which already encodes the project-scoped paths.