Add unit tests for requireWriteScope middleware

[Copilot]

requireWriteScope lacked test coverage for its three distinct code paths: PKCE tokens with scope=read being rejected, scope=write being allowed, and non-OAuth auth methods bypassing the scope check entirely.

Changes

• api/auth_middleware_test.go: Adds TestRequireWriteScope with three sub-tests covering each path. Since requireWriteScope only reads c.Locals("oauthScope") and never touches the receiver, tests use a bare &ApiServer{} — no DB required.

testApp.Post("/write", func(c *fiber.Ctx) error {
    if scope := c.Get("X-Test-Oauth-Scope"); scope != "" {
        c.Locals("oauthScope", scope)
    }
    return c.Next()
}, app.requireWriteScope, ...)

// read scope → 403
// write scope → 200
// no scope (non-OAuth) → 200

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